lztext and compression ratios...

Jeffery Collins <collins@onyx-technologies.com> writes:

My experience with attempting to compress such a relatively small
(around 1K) text string is that the compression ration is not very
good. This is because the string is not long enough for the LZ
compression algorithm to establish really good compression patterns and
the fact that the de-compression table has to be built into each
record. What I have done in the past to get around these problems is
that I have "taught" the compression algorithm the patterns ahead of
time and stored the de-compression patterns in an external table. Using
this technique, I have achieved *much* better compression ratios.

(Puts on compression-guru hat...)

There is much in what you say. Perhaps we should consider keeping the
lztext type around (currently it's slated for doom in 7.1, since the
TOAST feature will make plain text do everything lztext does and more)
and having it be different from text in that a training sample is
supplied when the column is defined. Not quite sure how that should
look or where to store the sample, but it could be a big win for tables
having a large number of moderate-sized text entries.

regards, tom lane

I was sitting here reading about TOAST and 7.1 and another question came
into my mind.. I looked on the web page for some information about features
to be added in 7.1 and an approximate 7.1 release date.. I found information
on the TOAST feature as well as Referential Integrity but nothing else.. If
I missed it please just point me in the right direction, if not I'd like to
as about the approximate time of 7.1 arriving in a stable form and if full
text indexing is going to be a part of 7.1..

I ask because if 7.1 is going to include full text indexing natively and is
going to arrive pretty soon, I might not continue on this project as I have
been (the new full text index trigger)..

Thanks!!!

-Mitch

figure sometime in October before v7.1 is released ... i believe the last
talk talked about a beta starting sometime in September, but nothing is
ever "set in stone" until we actually do it :)

On Wed, 5 Jul 2000, Mitch Vincent wrote:

I was sitting here reading about TOAST and 7.1 and another question came
into my mind.. I looked on the web page for some information about features
to be added in 7.1 and an approximate 7.1 release date.. I found information
on the TOAST feature as well as Referential Integrity but nothing else.. If
I missed it please just point me in the right direction, if not I'd like to
as about the approximate time of 7.1 arriving in a stable form and if full
text indexing is going to be a part of 7.1..

I ask because if 7.1 is going to include full text indexing natively and is
going to arrive pretty soon, I might not continue on this project as I have
been (the new full text index trigger)..

Thanks!!!

-Mitch

Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy
Systems Administrator @ hub.org
primary: scrappy@hub.org secondary: scrappy@{freebsd|postgresql}.org

"Mitch Vincent" <mitch@venux.net> writes:

I was sitting here reading about TOAST and 7.1 and another question came
into my mind.. I looked on the web page for some information about features
to be added in 7.1 and an approximate 7.1 release date.. I found information
on the TOAST feature as well as Referential Integrity but nothing else.. If
I missed it please just point me in the right direction, if not I'd like to
as about the approximate time of 7.1 arriving in a stable form and if full
text indexing is going to be a part of 7.1..

AFAIK there are no plans on the table to do more with full text
indexing; at least none of the core developers are working on it.
(If someone else is and I've forgotten, my apologies.)

Currently the plans for 7.1 look like this:

* WAL (assuming Vadim gets it done soon)
* TOAST (pretty far along already)
* new fmgr (fmgr itself done, still need to turn crank on
converting built-in functions)
* better memory management, no more intraquery memory leaks
(about half done)
* some other stuff, but I think those are all the "big features"
that anyone has committed to get done
* whatever else gets done meanwhile

We were shooting for going beta around Aug 1 with release around Sep 1,
but don't hold us to that ;-).

Notably missing from this list is OUTER JOIN and other things that
depend on querytree redesign (such as fixing all the restrictions on
views). We've agreed to put that off till 7.2 in hopes of keeping the
7.1 development cycle reasonably short. Not sure what else will be
in 7.2, but the querytree work will be.

I ask because if 7.1 is going to include full text indexing natively and is
going to arrive pretty soon, I might not continue on this project as I have
been (the new full text index trigger)..

Seems like you should keep at it.

regards, tom lane

Mitch Vincent wrote:

I was sitting here reading about TOAST and 7.1 and another question came
into my mind.. I looked on the web page for some information about features
to be added in 7.1 and an approximate 7.1 release date.. I found information
on the TOAST feature as well as Referential Integrity but nothing else.. If
I missed it please just point me in the right direction, if not I'd like to
as about the approximate time of 7.1 arriving in a stable form and if full
text indexing is going to be a part of 7.1..

The project page you've seen was an idea I had several month
ago. I had a big success with it since I got co-developers
for FOREIGN KEY. It was the first time I something in an
open source project with related developers, and the result
was stunning - even for me.

Unfortunately, none of the other "inner-circle" developers
catched up that idea. Maybe all they're doing is not of the
nature that they would gain anything from co-
developers/volunteers.

Not an answer to your real question, more like a kick in the
A.. of my colleagues.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

Jeffery Collins wrote:

I have been looking at using the lztext type and I have some
questions/observations. Most of my experience comes from attempting to
compress text records in a different database (CTREE), but I think the
experience is transferable.

First of all I welcome any suggestions/input to the
compression code I've implemented. Seems you're experienced
in this area so keep going and hammer down any of my points
below.

You should know that the "lztext" type will disappear soon
and be replaced with the general "all varlena types are
potentially compressable" approach of TOAST.

My typical table consists of variable length text records. The average
length record is around 1K bytes. I would like to compress my records
to save space and improve I/O performance (smaller records means more
records fit into the file system cache which means less I/O - or so the
theory goes). I am not too concerned about CPU as we are using a 4-way
Sun Enterprise class server. So compress seems like a good idea to me.

My experience with attempting to compress such a relatively small
(around 1K) text string is that the compression ration is not very
good. This is because the string is not long enough for the LZ
compression algorithm to establish really good compression patterns and
the fact that the de-compression table has to be built into each
record. What I have done in the past to get around these problems is
that I have "taught" the compression algorithm the patterns ahead of
time and stored the de-compression patterns in an external table. Using
this technique, I have achieved *much* better compression ratios.

The compression algorithm used in "lztext" (and so in TOAST
in the future) doesn't have a de-compression table at all.
It's based on Adisak Pochanayon's SLZ algorithm, using
<literal_char> or <token>. A <token> just tells how far to
go back in the OUTPUT-buffer and how many bytes to copy from
OUTPUT to OUTPUT. Look at the code for details.

My design rules for the compression inside of Postgres have
been

- beeing fast on decompression
- beeing good for relatively small values
- beeing fast on compression

The first rule is met by the implementation itself. Don't
underestimate this design rule! Usually you don't update as
often as you query. And the implementation of TOAST requires
a super fast decompression.

The second rule is met by not needing any initial
decompression table inside of the stored value.

The third rule is controlled by the default strategy of the
algorithm, (unfortunately) hardcoded into
utils/adt/pg_lzcompress.c. It'll never try to compress items
smaller than 256 bytes. It'll fallback to plain storage (for
speed advantage while decompressing a value) if less than 20%
of compression is gained. It'll stop match loookup if a
backward match of 128 or more bytes is found.

So my questions/comments are:

- What are the typical compression rations on relatively small (i.e.
around 1K) strings seen with lztext?

Don't have that small items handy. But a table containing a
file path and it's content. All files where HTML files.

From - To | count(*) | avg(length) | avg(octet_length)
---------------+----------+-------------+------------------
1024 - 2047 | 14 | 1905 | 1470
2048 - 4095 | 67 | 3059 | 1412
4096 - 8191 | 45 | 5384 | 2412
8192 - | 25 | 17200 | 6323
---------------+----------+-------------+------------------
all | 151 | 5986 | 2529

- Does anyone see a need/use for a generalized string compression
type that can be "trained" external to the individual records?

Yes, of course. Maybe "lztext" can be a framework for you and
we just tell the toaster "never apply your lousy compression
on that" (it's prepared for).

- Am I crazy in even attempting to compress strings of this relative
size? My largest table correct contains about 2 million entries of
roughly 1k size strings or about 2Gig of data. If I could compress this
to about 33% of it's original size (not unreasonable with a trained LZ
compression), I would save a lot of disk space (not really important)
and a lot of file system cache space (very important) and be able to fit
the entire table into memory (very, very important).

Noone is crazy attempting to improve something. It might turn
out not to work well, or beeing brain damaged from the start.
But someone who never tries will miss all his chances.

Final note:

One thing to keep in mind is that the LZ algorithm you're
thinking of must be distributable under the terms of the BSD
license. If it's copyrighted or patented by any third party,
not agreeing to these terms, it's out of discussion and will
never appear in the Postgres source tree. Especially the LZ
algorithm used in GIF is one of these show-stoppers.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

JanWieck@t-online.de (Jan Wieck) writes:

One thing to keep in mind is that the LZ algorithm you're
thinking of must be distributable under the terms of the BSD
license. If it's copyrighted or patented by any third party,
not agreeing to these terms, it's out of discussion and will
never appear in the Postgres source tree. Especially the LZ
algorithm used in GIF is one of these show-stoppers.

As long as you brought it up: how sure are you that the method you've
used is not subject to any patents? The mere fact that you learned
it from someone who didn't patent it does not guarantee anything ---
someone else could have invented it independently and filed for a
patent.

If you can show that this method uses no ideas not found in zlib,
then I'll feel reassured --- a good deal of legal research went into
zlib to make sure it didn't fall foul of any patents, and zlib has
now been around long enough that it'd be tough for anyone to get a
new patent on one of its techniques. But if SLZ has any additional
ideas in it, then we could be in trouble. There are an awful lot of
compression patents.

regards, tom lane

Jan,

Thank you for your comments on lztext. They were very informative. I hope
you (and the rest of the list) don't mind my ramblings on compression, but I
think there is some performance/space advantage if compression can integrated
into a table. I admit to not being a compression expert. All that I know
has been learn from reading some zlib source.

The compression that I have been working with (and am more familiar with) is
Huffman compression. You are probably familiar with it and may have even
evaluated it when you were looking at compression techniques. In my limited
understanding of LZ compression, there are some advantages and disadvantages
to Huffman compression.

The disadvantages seem to be:
- In normal Huffman compression, the string to be compressed is examined
twice. Once to build the translation table and once to do the translation.
- The translation table must be written with the string. This makes the
result larger and limits it's effectiveness with small strings.
- I don't know for sure, but I wouldn't be surprised if uncompression is
slower with Huffman than LZ compression.

To get around these disadvantages, I modified my Huffman support in the
following way:
- I trained the translation table and stored the table separately. I was
able to do this because I knew before hand the type of text that would be in
my columns.
- I "improved" the compression by giving the translation table the
ability to look for well known substrings in addition to byte values. For
example the string "THE" in an English text might have it's own Huffman
translation value instead of relying on the individual T, H and E
translations. Again I was able to do this because I knew the type of text I
would be storing in the column.

Because the translation table is external to the string, it is no longer
included in the string. This improves the compression ratio and helps the
uncompression as the table does not need to be read and interpreted. This
approach also allows for and takes advantage of cross-row commonalities.
After doing all of this, I was able to get the average compression ratio to
about 3.5 (i.e. orig size / new size) on text columns of 1K or less.

Of course the disadvantage to this is that if the dataset changes overtime,
the compression translations to not change with it and the compression ratio
will get worse. In the very worst case, where the dataset totally changes
(say the language changed from English to Russian) the "compressed" string
could actually get larger than the original string because everything the
compression algorithm thought it knew was totally wrong.

As I said, I did this with a different database (CTREE). My company is now
converting from CTREE to postgresql and I would like to find some fairly
elegant way to include this type of compression support. My CTREE
implementation was rather a hack, but now that we are moving to a better
database, I would like to have a better solution for compression.

Originally I latched onto the idea of using lztext or something like lztext,
but I agree it is better if the column type is something standard and the
compression is hidden by the backend. I guess my vision would be to be able
to define either a column type or a column attribute that would allow me to
indicate the type of compression to have and possibly the compression
algorithm and translation set to use. Actually the real vision is to have it
all hidden and have the database learn about the column as rows are added and
modify the compression transalation to adapt to the changing column data.

Jeff

Tom Lane wrote:

JanWieck@t-online.de (Jan Wieck) writes:

One thing to keep in mind is that the LZ algorithm you're
thinking of must be distributable under the terms of the BSD
license. If it's copyrighted or patented by any third party,
not agreeing to these terms, it's out of discussion and will
never appear in the Postgres source tree. Especially the LZ
algorithm used in GIF is one of these show-stoppers.

As long as you brought it up: how sure are you that the method you've
used is not subject to any patents? The mere fact that you learned
it from someone who didn't patent it does not guarantee anything ---
someone else could have invented it independently and filed for a
patent.

Now that you ask for it: I'm not sure. Could be.

If you can show that this method uses no ideas not found in zlib,
then I'll feel reassured --- a good deal of legal research went into
zlib to make sure it didn't fall foul of any patents, and zlib has
now been around long enough that it'd be tough for anyone to get a
new patent on one of its techniques. But if SLZ has any additional
ideas in it, then we could be in trouble. There are an awful lot of
compression patents.

To do so I don't know enough about the algorithms used in
zlib. Is there someone out here who could verify that if I
detailed enough describe what our compression code does?

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

JanWieck@t-online.de (Jan Wieck) writes:

As long as you brought it up: how sure are you that the method you've
used is not subject to any patents?

Now that you ask for it: I'm not sure. Could be.

If you can show that this method uses no ideas not found in zlib,
then I'll feel reassured

To do so I don't know enough about the algorithms used in
zlib. Is there someone out here who could verify that if I
detailed enough describe what our compression code does?

After a quick look at the code, I don't think there is anything
problematic about the data representation or the decompression
algorithm. The compression algorithm is another story, and it's
not real well commented :-(. The important issues are how you
search for matches in the past text and how you decide which match
is the best one to use. Please update the code comments to describe
that, and I'll take another look.

regards, tom lane

Tom Lane wrote:

JanWieck@t-online.de (Jan Wieck) writes:

As long as you brought it up: how sure are you that the method you've
used is not subject to any patents?

Now that you ask for it: I'm not sure. Could be.

If you can show that this method uses no ideas not found in zlib,
then I'll feel reassured

To do so I don't know enough about the algorithms used in
zlib. Is there someone out here who could verify that if I
detailed enough describe what our compression code does?

After a quick look at the code, I don't think there is anything
problematic about the data representation or the decompression
algorithm. The compression algorithm is another story, and it's
not real well commented :-(. The important issues are how you
search for matches in the past text and how you decide which match
is the best one to use. Please update the code comments to describe
that, and I'll take another look.

Done. You'll find a new section in the top comments.

While writing it I noticed that the algorithm is really
expensive for big items. The history lookup table allocated
is 8 times (on 32 bit architectures) the size of the input.
So if you want to have 1MB compressed, it'll allocate 8MB for
the history. It hit me when I was hunting a bug in the
toaster earlier today. Doing an update to a toasted item of
5MB, resulting in a new value of 10MB, the backend blew up to
290MB of virtual memory - oh boy. I definitely need to make
that smarter.

When I wrote it I never thought about items that big. It was
before we had the idea of TOAST.

This all might open another discussion I'll start in a
separate thread.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

figure sometime in October before v7.1 is released ... i believe the last
talk talked about a beta starting sometime in September, but nothing is
ever "set in stone" until we actually do it :)

I agree. August is a very useful month because a lot of people are slow
at work during that month, and that gives them time to work on
PostgreSQL.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

JanWieck@t-online.de (Jan Wieck) writes:

Tom Lane wrote:

After a quick look at the code, I don't think there is anything
problematic about the data representation or the decompression
algorithm. The compression algorithm is another story, and it's
not real well commented :-(. The important issues are how you
search for matches in the past text and how you decide which match
is the best one to use. Please update the code comments to describe
that, and I'll take another look.

Done. You'll find a new section in the top comments.

I think we are probably OK. Anyone who wants to check the issue might
want to start with http://www.faqs.org/faqs/compression-faq/,
particularly part 1 item 8. The two patents I'm most concerned about
are Waterworth's (4701745) and Stac's (5016009) which both cover basic
variants of LZ77 (for full text of these or any other US patent consult
http://patent.womplex.ibm.com/). But it looks to me like this
particular variant can be argued not to fall under either patent.
For example, Waterworth's patent specifies using 3-byte hash values
while you use 4-byte, and stupid as that is it's sufficient to get
you out from under. (A more technically valid reason is that he
doesn't use hash collision chains.) The Stac patent also specifies
a data structure considerably different from your hash chains.

To give you an idea of what's involved here, I attach an old message
from Jean-loup Gailly, author of gzip and zlib, explaining why he
believes gzip doesn't fall foul of these two patents. Not all of
his reasons apply to your method, but I think enough do. (BTW,
Gailly's a good guy --- I worked with him on the PNG project.
I think I'll write to him and see if he's got time to review this
code for patent problems.)

While writing it I noticed that the algorithm is really
expensive for big items. The history lookup table allocated
is 8 times (on 32 bit architectures) the size of the input.
So if you want to have 1MB compressed, it'll allocate 8MB for
the history. It hit me when I was hunting a bug in the
toaster earlier today. Doing an update to a toasted item of
5MB, resulting in a new value of 10MB, the backend blew up to
290MB of virtual memory - oh boy. I definitely need to make
that smarter.

Yes, you need some smarter method for choosing the size of the
hash table... a braindead but possibly sufficient answer is to
have some hard limit on the size... or you could just use a
hard-wired constant size to begin with. I think it's usually
considered wise to use a prime for the table size and reduce
the raw input bits modulo the prime.

regards, tom lane

Article: 14604 of gnu.misc.discuss
Path: chorus!octave.chorus.fr!jloup
From: jloup@chorus.fr (Jean-loup Gailly)
Newsgroups: gnu.misc.discuss
Subject: Re: LPF, NY Times, GNU and compression algorithms
Message-ID: <7989@chorus.chorus.fr>
Date: 10 Mar 94 10:10:03 GMT
References: <RXN106.94Mar6103732@wilbur.cac.psu.edu> <HSU.94Mar7204324@laphroaig.cs.hut.fi> <MARC.94Mar8090631@marc.watson.ibm.com>
Sender: news@chorus.chorus.fr
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Marc Auslander <marc@watson.ibm.com> writes:

Two Stac patents in the case are 4701745 and 5016009.

From 4601745: [typo, meant: 4701745]

the data processing means including circuit means operable to check
whether a sequence of successive bytes to be processed identical with
a sequence of bytes already processed, and including a hash generating
means responsive to the application of a predetermined number of bytes ...

From 5016009

in order to perform the hashing function, a data compression system
includes certain hash data structures including a history array
pointer ...

also:

... is found ..., encoding said matching string ... a variable length
indicator ..., said predetermined strategy ensuring that a matching
string of two characters of said input data stream is compressed to
less than said two characters of said input data stream.

So - on the face of it, why isn't gzip covered by these patents?

Let's take each patent in turn. A clarification first: the Stac patent
4,701,745 was not invented by Stac. It was initially owned by Ferranti
(UK) and only recently bought by Stac. This was a very profitable
acquisition (assuming it cost Stac far less than the $120 million they
won by using this patent against Microsoft).

(a) 4,701,745

This algorithm is now known as LZRW1, because Ross Williams reinvented
it independently later and posted it on comp.compression on April 22,
1991. Exactly the same algorithm has also been patented by Gibson and
Graybill (5,049,881). The patent office failed to recognize that
it was the same algorithm.

This algorithm uses LZ77 with hashing but no collision chains and
outputs unmatched bytes directly without further compression. gzip
uses collisions chains of arbitrary length, and uses Huffman encoding
on the unmatched bytes:

- Claim 1 of the patent is restricted to (emphasis added by me):

output means operable to APPLY to a transfer medium each byte of data
not forming part of such an identical sequence; and

ENCODING means responsive to the identification of such a sequence
to APPLY to the transfer medium an identification signal which
identifies both the location in the input store of the previous
occurrence of the sequence of bytes and the number of bytes
contained in the sequence.

The claim thus makes a clear distinction between "encoding" and
"applying to the transfer medium". A system which compresses the
unmatched bytes does not infringe this patent.

- The description of the patent and claim 2 make clear that the check
for identity of the sequences of bytes is to be made only once (no
hash collision chains). Gzip performs an arbitrary number of such
checks. The "means" enumerated in claim 1 specify what the hash
table consists of, and this does not include any means for storing
hash collision chains.

- Claim 2 also requires that *all* bytes participating in the hash
function should be compared:

A system as claimed in claim 1 in which the circuit means also
includes check means operable to check for identity between EACH
of the said predetermined number of bytes in sequence and EACH of
a similar sequence of bytes contained in the input store at a
location defined by a pointer read out from the temporary store at
said address

[in plain English, this is the check for hash collision]

and to check whether identity exists between succeeding bytes in
each sequence of bytes, and a byte counter operable to count the
number of identical bytes in each sequence.

[this is the determination of the match length]

Gzip never checks for equality of the third byte used in the hash
function. The hash function is such that on a hash hit with equality
of the first two bytes, the third byte necessarily matches.

- In addition, gzip uses a "lazy" evaluation of string matches. Even
when a match is found, gzip may encode (with Huffman coding) a single
unmatched byte. This is done when gzip determines that it is more
beneficial to parse the input string differently because a longer
match follows. In the Waterworth patent, a string match is always
encoded as a (length, pointer) pair.

All other claims of the patent are dependent on claim 1
("a system as claimed in claim 1 in which ..."). Since gzip
does not infringe claim 1 it does not infringe the other claims.
In particular, claim 6 explicitly states that unmatched strings
are not compressed:

A system as claimed in claim 5 in which the data receiving means
includes decoder means operable to separate UNCOMPRESSED bytes of
data from identification signals.

The gzip decoder never receives uncompressed bytes since all input is
compressed with Huffman coding [both literals and (length, offset) pairs].

The only "invention" in the Waterworth patent is the absence of hash
collision chains. The original description of the LZ77 algorithm
required searching for the true longest match, not just checking the
length of one particular match. Using hashing for string searching
was very well known at the time of the patent application (March 86).
The patent description specifies explicitly that "Hash techniques are
well known and many differents forms of hash will be suitable".

The --fast option of gzip was on purpose made slower than possible
precisely because of the existence of the Waterworth patent.
See in particular the following part of the gzip TODO file:

Add a super-fast compression method, suitable for implementing
file systems with transparent compression. One problem is that the
best candidate (lzrw1) is patented twice (Waterworth 4,701,745
and Gibson & Graybill 5,049,881).

(b) 5,016,009

This is standard LZ77 with hashing, and collisions resolved using
linked lists. There are several important restrictions which let gzip
escape from the patent:

- the linked lists are implemented only with offsets. The end of
a chain is detected by adding together all offsets, until the
sum becomes greater than the size of the history buffer. gzip uses
direct indices, and so detects the end of the chains differently.
The exact wording of claim 1 of the patent is:

... said data compression system comprising ... an offset array means
... said method comprising the steps of ...

calculating a difference between said history array pointer
and said pointer obtained from said hash table means,
storing said difference into said offset array means entry
pointed to by said history array pointer, ...

gzip never calculates such a difference and does not have any offset
array.

- unmatched strings are emitted as literal bytes without any
compression. gzip uses Huffman encoding on the unmatched strings.
This is the same argument as for the Waterworth patent.

- unmatched strings are preceded by
... a "raw" data tag indicating that no matching data string was found

gzip does not use such a tag because it uses a single Huffman table for
both string literals and match lengths. It is only the prefix
property of Huffman codes which allows the decoder to distinguish
the two cases. So there is not a unique "raw" tag preceding all
literals. This is not a minor point. It is one of the reasons
giving gzip superior compression to that obtained with the Stac
algorithm.

- a string match is always encoded as a (length, pointer) pair.
Gzip uses a "lazy" evaluation of string matches as described
above for the Waterworth patent.

All other claims of the patent are dependent on claim 1 ("the method
of claim 1 wherein ..."). Since gzip does not infringe claim 1 it does
not infringe the other claims. In any case, I have studied in detail
all the 77 claims to make sure that gzip does not infringe.

Unrelated note: this Stac patent is the only one where I found an
original and non obvious idea. The hash table is refreshed using a
incremental mechanism, so that the refresh overhead is distributed
among all input bytes. This allows the real response time necessary in
disk compressors such as Stacker (the Stac product). gzip does not use
this idea, and refreshes the hash table in a straightforward manner
every 32K bytes.

One final comment: I estimate that I have now spent more time studying
data compression patents than actually implementing data compression
algorithms. I have a partial list of 318 data compression patents,
even though for the moment I restrict myself mostly to lossless
algorithms (ignoring most image compression patents). Richard
Stallman has been *extremely* careful before accepting gzip as the GNU
compressor. I continue to study new patents regularly. I would of
course very much prefer spending what's left of my spare time
improving the gzip compression algorithm instead of studying patents.
Some improvements that I would have liked to put in gzip have not been
incorporated because of patents.

In short, every possible precaution has been taken to make sure that
gzip isn't covered by patents.

Jean-loup Gailly, author of gzip.
jloup@chorus.fr

Maybe you just want to use zlib. Let other guys hammer out the details.

On Thu, 6 Jul 2000, Jan Wieck wrote:

While writing it I noticed that the algorithm is really
expensive for big items. The history lookup table allocated
is 8 times (on 32 bit architectures) the size of the input.
So if you want to have 1MB compressed, it'll allocate 8MB for
the history. It hit me when I was hunting a bug in the
toaster earlier today. Doing an update to a toasted item of
5MB, resulting in a new value of 10MB, the backend blew up to
290MB of virtual memory - oh boy. I definitely need to make
that smarter.

When I wrote it I never thought about items that big. It was
before we had the idea of TOAST.

This all might open another discussion I'll start in a
separate thread.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

--
Peter Eisentraut Sernanders vaeg 10:115
peter_e@gmx.net 75262 Uppsala
http://yi.org/peter-e/ Sweden

eisentrp@csis.gvsu.edu writes:

Maybe you just want to use zlib. Let other guys hammer out the details.

I've been wondering about that myself. Jan, have you actually
benchmarked decompression speed on this method vs. stock zlib?
Unless the differential is huge we might want to just skip the
patent worries.

zlib has a BSD-style license, so we could include it in our
distribution to avoid worries about whether it's installed.

I've written to Jean-loup for his advice. He's pretty busy these
days (CTO at some startup, I believe) but we'll see if he has time
to comment.

regards, tom lane

eisentrp@csis.gvsu.edu writes:

Maybe you just want to use zlib. Let other guys hammer out the details.

I've been wondering about that myself. Jan, have you actually
benchmarked decompression speed on this method vs. stock zlib?
Unless the differential is huge we might want to just skip the
patent worries.

If we found later that there was a patent problem, could we just issue a
new release with a new compression algorithm?

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

Bruce Momjian <pgman@candle.pha.pa.us> writes:

Unless the differential is huge we might want to just skip the
patent worries.

If we found later that there was a patent problem, could we just issue a
new release with a new compression algorithm?

Yeah, we could, and it could presumably even be a fully compatible
dot-release with no change to the on-disk representation. That
representation and consequently the decompression algorithm are safe
enough, it's the details of the compressor's search for matching
patterns that are a patent minefield.

However, changing the code after-the-fact might not be enough to keep us
from being sued :-(. I'd rather use something that's pretty well
established as being in the clear...

regards, tom lane

eisentrp@csis.gvsu.edu wrote:

Maybe you just want to use zlib. Let other guys hammer out the details.

We cannot assume that zlib is available everywhere. Thus it
must be determined during configure and where it isn't, TOAST
can only move off values to make tuples fit into blocks.
Since decompression of already in memory items is alot faster
than doing an index scan on the TOAST table, I expect this to
make installations without zlib damned slow.

And how should binary distributions like RPM's handle it? I
assume that this problem is already on it's way because of
the integration of zlib into pg_dump. The only way I see is
having different RPM's for each possible combination of
available helper libs. Or is there another way to work
around?

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

JanWieck@t-online.de (Jan Wieck) writes:

eisentrp@csis.gvsu.edu wrote:

Maybe you just want to use zlib. Let other guys hammer out the details.

We cannot assume that zlib is available everywhere.

We can if we include it in our distribution --- which we could; it's
pretty small and uses a BSD-style license. I can assure you the zlib
guys would be happy with that. And it's certainly as portable as our
own code. The real question is, is a custom compressor enough better
than zlib for our purposes to make it worth taking any patent risks?

We could run zlib at a low compression setting (-z1 to -z3 maybe)
to make compression relatively fast, and since that also doesn't
generate a custom Huffman tree, the overhead in the compressed data
is minor even for short strings. And its memory footprint is
certainly no worse than Jan's method...

The real question is whether zlib decompression is markedly slower
than Jan's code. Certainly Jan's method is a lot simpler and *should*
be faster --- but on the other hand, zlib has had a heck of a lot
of careful performance tuning put into it over the years. The speed
difference might not be as bad as all that.

I think it's worth taking a look at the option.

regards, tom lane

Jan Wieck writes:

Maybe you just want to use zlib. Let other guys hammer out the details.

And how should binary distributions like RPM's handle it?

They got all their fancy dependency mechanisms for that (which never work
well, but anyway).

--
Peter Eisentraut Sernanders v�g 10:115
peter_e@gmx.net 75262 Uppsala
http://yi.org/peter-e/ Sweden

At 21:47 7/07/00 +0200, Jan Wieck wrote:

And how should binary distributions like RPM's handle it? I
assume that this problem is already on it's way because of
the integration of zlib into pg_dump. The only way I see is
having different RPM's for each possible combination of
available helper libs. Or is there another way to work
around?

This remoinded me of a question I wanted to ask Unix people: other OSs I
use allow for dynamic linking, at runtime and in code, against shared
libraries, and I know Unix must allow this. The places where zlib is used
are pretty limited, so it might be worth considering doing the 'HAVE_ZLIB'
kinds of checks at runtime. Then one binary fits all...

Is this hard or easy - at least on machines with a libz.so?

Is it worth doing?

I guess the alternative on rpm is to create both: pg_dump.zlib and
pg_dump.nozlib, and install the right one?

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.C.N. 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/

This remoinded me of a question I wanted to ask Unix people: other OSs I
use allow for dynamic linking, at runtime and in code, against shared
libraries, and I know Unix must allow this. The places where zlib is used
are pretty limited, so it might be worth considering doing the 'HAVE_ZLIB'
kinds of checks at runtime. Then one binary fits all...

Is this hard or easy - at least on machines with a libz.so?

Is it worth doing?

I guess the alternative on rpm is to create both: pg_dump.zlib and
pg_dump.nozlib, and install the right one?

We do dynamic loading for functions. Not sure if we want to load zlib
dynamically if we can help it.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

This remoinded me of a question I wanted to ask Unix people: other OSs I
use allow for dynamic linking, at runtime and in code, against shared
libraries, and I know Unix must allow this. The places where zlib is used
are pretty limited, so it might be worth considering doing the 'HAVE_ZLIB'
kinds of checks at runtime. Then one binary fits all...

We do dynamic loading for functions. Not sure if we want to load zlib
dynamically if we can help it.

Not bloody likely! Do you want to be in a position where you restart
your postmaster and suddenly chunks of your database are inaccessible?
That's what could happen to you if someone moves or deletes libz.so.

I don't mind being dynamically linked to standard system shared libs;
if libc.so is busted then whether Postgres launches is the least of
your worries. But dynamic dependence on an optional package that's
probably living in /usr/local strikes me as exceedingly risky.

If we do go with using zlib instead of homegrown code, I would recommend
building and statically linking to our own copy even if there is a copy
available on the system. This will prevent cross-version compatibility
problems as well as where'd-my-library-go syndrome. We cannot afford
those sorts of risks for something that could prevent us from reading
our database.

regards, tom lane

If we do go with using zlib instead of homegrown code, I would recommend
building and statically linking to our own copy even if there is a copy
available on the system. This will prevent cross-version compatibility
problems as well as where'd-my-library-go syndrome. We cannot afford
those sorts of risks for something that could prevent us from reading
our database.

I don't know much about zlib, but it is hard to imagine it would have
the flexibility and optimization tuning of Jan's code.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

At 21:49 7/07/00 -0400, Tom Lane wrote:

Not bloody likely! Do you want to be in a position where you restart
your postmaster and suddenly chunks of your database are inaccessible?
That's what could happen to you if someone moves or deletes libz.so.

My question was limited to it's use in pg_dump; rather than basing
pg_dump's compression bahaviour on configure, base it on it's runtime
environment. But my guess is you still would be inclined, rather strongly,
against it.

If we do go with using zlib instead of homegrown code

This begs the obvious question: should pg_dump be using Jan's compression
code? In all cases/when zlib is not available?

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.C.N. 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/

Bruce Momjian <pgman@candle.pha.pa.us> writes:

I don't know much about zlib, but it is hard to imagine it would have
the flexibility and optimization tuning of Jan's code.

Uh, I think you have that backwards ...

regards, tom lane

PS: I do know a few things about zlib.

Philip Warner <pjw@rhyme.com.au> writes:

Not bloody likely! Do you want to be in a position where you restart
your postmaster and suddenly chunks of your database are inaccessible?
That's what could happen to you if someone moves or deletes libz.so.

My question was limited to it's use in pg_dump; rather than basing
pg_dump's compression bahaviour on configure, base it on it's runtime
environment. But my guess is you still would be inclined, rather strongly,
against it.

pg_dump is rather a different case. I would want to see a runtime
option *not* to use compression, in case you know you are going to
need to restore on another system where zlib (or whatever) isn't
available. But making an uncompressed dump today doesn't invalidate
the compressed dump you made yesterday, nor vice versa.

If we do go with using zlib instead of homegrown code

This begs the obvious question: should pg_dump be using Jan's compression
code? In all cases/when zlib is not available?

Good point. If we include zlib in the distribution it would be pretty
silly for pg_dump not to use it. If we don't, then Peter's remarks
about not liking an environment-determined feature set are relevant.

regards, tom lane

At 22:28 7/07/00 -0400, Tom Lane wrote:

pg_dump is rather a different case. I would want to see a runtime
option *not* to use compression

That's there already; using -Z0 produces the same output as a mchine
without zlib.

But making an uncompressed dump today doesn't invalidate
the compressed dump you made yesterday, nor vice versa.

Not sure what you mean here.

Good point. If we include zlib in the distribution it would be pretty
silly for pg_dump not to use it. If we don't, then Peter's remarks
about not liking an environment-determined feature set are relevant.

zlib seems to have been ported to zillions of architectures (must be a good
design ;-}), so it's probably pretty safe to put in the source tree, and
I'm sure any porting problems we encounter would be useful to the zlib
maintainers.

If we don't want to use zlib, then fefore using Jan's compression code in
pg_dump, I suppose I'd need to know that the output is compatible across
32/64 bit architectures and is not sensitive to other issues like endian-ness.

----------------------------------------------------------------
Philip Warner | __---_____
Albatross Consulting Pty. Ltd. |----/ - \
(A.C.N. 008 659 498) | /(@) ______---_
Tel: (+61) 0500 83 82 81 | _________ \
Fax: (+61) 0500 83 82 82 | ___________ |
Http://www.rhyme.com.au | / \|
| --________--
PGP key available upon request, | /
and from pgp5.ai.mit.edu:11371 |/

Peter Eisentraut wrote:

And how should binary distributions like RPM's handle it?

They got all their fancy dependency mechanisms for that (which never work
well, but anyway).

Debian's work very well, because the Debian repository is centralised
under uniform quality control.

As a package maintainer, I can allow a package to depend on zlib -- in
which case the dependency will be satisfied by any version -- or I can
specify a versioned dependency. I might have to provide the correct
library myself, if the zlib maintainer didn't want to maintain an old
version. At the moment Debian provides zlib1g; if the major version
changes, I would expect to see this package remain and a new zlib2
package be created.

I would prefer a solution that did not _require_ me to maintain zlib
as well, if the dependency can be satisfied from another package.

--
Oliver Elphick Oliver.Elphick@lfix.co.uk
Isle of Wight http://www.lfix.co.uk/oliver
PGP: 1024R/32B8FAA1: 97 EA 1D 47 72 3F 28 47 6B 7E 39 CC 56 E4 C1 47
GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839 932A 614D 4C34 3E1D 0C1C
========================================
"Ask, and it shall be given you; seek, and ye shall
find; knock, and it shall be opened unto you."
Matthew 7:7

Philip Warner wrote:

At 21:49 7/07/00 -0400, Tom Lane wrote:

Not bloody likely! Do you want to be in a position where you restart
your postmaster and suddenly chunks of your database are inaccessible?
That's what could happen to you if someone moves or deletes libz.so.

My question was limited to it's use in pg_dump; rather than basing
pg_dump's compression bahaviour on configure, base it on it's runtime
environment. But my guess is you still would be inclined, rather strongly,
against it.

If we do go with using zlib instead of homegrown code

This begs the obvious question: should pg_dump be using Jan's compression
code? In all cases/when zlib is not available?

It can't. My code is designed for in-memory attribute values.
It doesn't support streaming - which I assume is required for
pg_dump.

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

Hi!

How can I count the number of rows affected by UPDATE or DELETE?

Przem

How can I count the number of rows affected by UPDATE or DELETE?

This depends on the interface you are using.

* The psql SQL monitor always reports the number of affected rows
(it's the 2nd number)

* In DBI "do" returns the number of affected rows.

* In JDBC "executeUpdate" does the same. (By the way, is there a way
to get the number of rows from a executeQuery without counting them?).

Regards,
Mit freundlichem Gru�,
Holger Klawitter
--
Holger Klawitter +49 (0)251 484 0637
holger@klawitter.de http://www.klawitter.de/

Tom Lane wrote:

JanWieck@t-online.de (Jan Wieck) writes:

eisentrp@csis.gvsu.edu wrote:

Maybe you just want to use zlib. Let other guys hammer out the details.

We cannot assume that zlib is available everywhere.

We can if we include it in our distribution --- which we could; it's
pretty small and uses a BSD-style license. I can assure you the zlib
guys would be happy with that. And it's certainly as portable as our
own code. The real question is, is a custom compressor enough better
than zlib for our purposes to make it worth taking any patent risks?

Good, we shouldn't worry about that anymore. If we want to
use zlib, I vote for including it into our distribution and
link static against the one shipped with our code.

If we want to ...

We could run zlib at a low compression setting (-z1 to -z3 maybe)
to make compression relatively fast, and since that also doesn't
generate a custom Huffman tree, the overhead in the compressed data
is minor even for short strings. And its memory footprint is
certainly no worse than Jan's method...

Definitely not, it's memory footprint is actually much
smaller. Thus, I need to recreate the comparision below
again after making the history table of fixed size with a
wrap around mechanism to get a small footprint on multi-MB
inputs too.

The real question is whether zlib decompression is markedly slower
than Jan's code. Certainly Jan's method is a lot simpler and *should*
be faster --- but on the other hand, zlib has had a heck of a lot
of careful performance tuning put into it over the years. The speed
difference might not be as bad as all that.

I think it's worth taking a look at the option.

Some quick numbers though:

I simply stripped down pg_lzcompress.c to call compress2()
and uncompress() instead of doing anything itself (what a
nice, small source file :-). There might be some room for
improvement using static zlib stream allocaions and
deflateReset(), inflateReset() or the like. But I don't
expect a significant difference from that.

The test is a Tcl (pgtclsh) script doing the following:

- Loading 151 HTML files into a table t1 of structure (path
text, content lztext).

- SELECT * FROM t1 and checking for correct result set.
Each file is read again during the check.

- UPDATE t1 SET content = upper(content).

� SELECT * FROM t1 and checking for correct result set.
Each file is read again, converted to upper case using
Tcl's "string toupper" function for comparision.

- SELECT path FROM t1. Loop over result set to UPDATE t1
SET content = <value> WHERE path = <path>. All files are
read again and converted to lower case before UPDATE.

- SELECT * FROM t1 and check for correct result set. Files
are again reread and lower case converted in Tcl for
comparision.

- Doing 20 SELECT * FROM t1 to have alot more decompress
than compress cycles.

Of course, there's an index on path. Here are the timings and
sizes:

Compressor | level | heap size | toastrel | toastidx | seconds
| | | size | size |
-----------+-------+-----------+----------+----------+--------
PGLZ | - | 425,984 | 950,272 | 32,768 | 5.20
zlib | 1 | 499,712 | 614,400 | 16,384 | 6.85
zlib | 3 | 499,712 | 557,056 | 16,384 | 6.75
zlib | 6 | 491,520 | 524,288 | 16,384 | 7.10
zlib | 9 | 491,520 | 524,288 | 16,384 | 7.21

Seconds is an average over multiple runs. Interesting is that
compression level 3 seems to be faster than 1. I double
checked it because it was so surprising.

Also, increasing the number of SELECT * at the end increases
the difference. So the PGLZ decompressor does a perfect job.

And what must be taken into account too is that the script,
running on the same processor and doing all the overhead
(reading files, doing case conversions, quoting values with
regsub and comparisions), along with the normal Postgres
query execution (parsing, planning, optimizing, execution)
occupies a substantial portion of the bare runtime. Still
PGLZ is about 25% faster than the best zlib compression level
I'm seeing, while zlib gains a much better compression ratio
(factor 1.7 at least).

As I see it:

If replacing the compressor/decompressor can cause a runtime
difference of 25% in such a scenario, the pure difference
between the two methods must be alot.

PGLZ is what I mentioned in the comments. Optimized for speed
on the cost of compression ratio.

What I suggest:

Leave PGLZ in place as the default compressor for toastable
types. Speed is what all benchmarks talk about - on disk
storage size is seldom a minor note.

Fix it's history allocation for huge values and have someone
(PgSQL Inc.?) patenting the compression algorithm, so we're
safe at some point in the future. If there's a patent problem
in it, we are already running the risk to get sued, the PGLZ
code got shipped with 7.0, used in lztext.

We can discuss about enabling zlib as a per attribute
configurable alternative further. But is the confusion this
might cause worth it all?

Jan

--

#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#================================================== JanWieck@Yahoo.com #

Compressor | level | heap size | toastrel | toastidx | seconds
| | | size | size |
-----------+-------+-----------+----------+----------+--------
PGLZ | - | 425,984 | 950,272 | 32,768 | 5.20
zlib | 1 | 499,712 | 614,400 | 16,384 | 6.85
zlib | 3 | 499,712 | 557,056 | 16,384 | 6.75
zlib | 6 | 491,520 | 524,288 | 16,384 | 7.10
zlib | 9 | 491,520 | 524,288 | 16,384 | 7.21

Consider that the 25% slowness gets us a 35% disk reduction, and that
translates to fewer buffer blocks and disk accesses. Seems there is a
clear tradeoff there.

If replacing the compressor/decompressor can cause a runtime
difference of 25% in such a scenario, the pure difference
between the two methods must be alot.

Leave PGLZ in place as the default compressor for toastable
types. Speed is what all benchmarks talk about - on disk
storage size is seldom a minor note.

True, disk storage is not the issue, but disk access are an issue.

We can discuss about enabling zlib as a per attribute
configurable alternative further. But is the confusion this
might cause worth it all?

I think we have to choose one proposal.

-- 
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

Bruce Momjian <pgman@candle.pha.pa.us> writes:

Compressor | level | heap size | toastrel | toastidx | seconds
| | | size | size |
-----------+-------+-----------+----------+----------+--------
PGLZ | - | 425,984 | 950,272 | 32,768 | 5.20
zlib | 1 | 499,712 | 614,400 | 16,384 | 6.85
zlib | 3 | 499,712 | 557,056 | 16,384 | 6.75
zlib | 6 | 491,520 | 524,288 | 16,384 | 7.10
zlib | 9 | 491,520 | 524,288 | 16,384 | 7.21

Consider that the 25% slowness gets us a 35% disk reduction, and that
translates to fewer buffer blocks and disk accesses. Seems there is a
clear tradeoff there.

Also, consider that in the vast majority of cases, reads greatly outnumber
writes for any given datum, and particularly so for web applications.

Could we get a benchmark that compared decompression speeds exclusively?

-Michael Robinson

JanWieck@t-online.de (Jan Wieck) writes:

Some quick numbers though:
I simply stripped down pg_lzcompress.c to call compress2()
and uncompress() instead of doing anything itself (what a
nice, small source file :-).

I went at it in a different way: pulled out pg_lzcompress into a
standalone source program that could also call zlib. These numbers
represent the pure compression or decompression time for memory-to-
memory processing, no other overhead at all. Each run was iterated
1000 times to make it long enough to time accurately (so you can
read the times as "milliseconds per operation", though they're
really seconds).

A small text file of about 1K:

LZ compressed 990 bytes to 717 bytes in 0.35 u 0.00 sys sec
LZ decompressed 717 bytes to 990 bytes in 0.04 u 0.00 sys sec
zlib(3) compressed 990 bytes to 527 bytes in 0.73 u 0.00 sys sec
zlib uncompressed 527 bytes to 990 bytes in 0.21 u 0.00 sys sec
zlib(6) compressed 990 bytes to 513 bytes in 0.86 u 0.00 sys sec
zlib uncompressed 513 bytes to 990 bytes in 0.21 u 0.00 sys sec
zlib(9) compressed 990 bytes to 513 bytes in 0.86 u 0.00 sys sec
zlib uncompressed 513 bytes to 990 bytes in 0.20 u 0.00 sys sec

A larger text file, a bit under 100K:

LZ compressed 92343 bytes to 54397 bytes in 49.00 u 0.05 sys sec
LZ decompressed 54397 bytes to 92343 bytes in 3.74 u 0.00 sys sec
zlib(3) compressed 92343 bytes to 40524 bytes in 38.48 u 0.02 sys sec
zlib uncompressed 40524 bytes to 92343 bytes in 8.58 u 0.00 sys sec
zlib(6) compressed 92343 bytes to 38002 bytes in 68.04 u 0.03 sys sec
zlib uncompressed 38002 bytes to 92343 bytes in 8.13 u 0.00 sys sec
zlib(9) compressed 92343 bytes to 37961 bytes in 71.99 u 0.03 sys sec
zlib uncompressed 37961 bytes to 92343 bytes in 8.14 u 0.00 sys sec

It looks like the ultimate speed difference is roughly 2:1 on
decompression and less than that on compression, though zlib suffers
from greater setup overhead that makes it look worse on short strings.

Here is a much different example, a small JPEG image:

LZ compressed 5756 bytes to 5764 bytes in 1.48 u 0.00 sys sec
LZ decompressed 5764 bytes to 5756 bytes in 0.01 u 0.00 sys sec
zlib(3) compressed 5756 bytes to 5629 bytes in 3.18 u 0.00 sys sec
zlib uncompressed 5629 bytes to 5756 bytes in 0.75 u 0.00 sys sec
zlib(6) compressed 5756 bytes to 5625 bytes in 3.78 u 0.00 sys sec
zlib uncompressed 5625 bytes to 5756 bytes in 0.75 u 0.00 sys sec
zlib(9) compressed 5756 bytes to 5625 bytes in 3.78 u 0.00 sys sec
zlib uncompressed 5625 bytes to 5756 bytes in 0.75 u 0.00 sys sec

The PG-LZ code realizes it cannot compress this file and falls back to
straight bytewise copy, yielding very fast "decompress". zlib manages
to eke out a few bytes' savings, so it goes ahead and compresses
instead of just storing literally, resulting in a big loss on
decompression speed for just a fractional space savings. If we use
zlib, we'd probably be well advised to accept a compressed result only
if it's at least, say, 25% smaller than uncompressed to avoid this
low-return-on-investment situation.

There might be some room for
improvement using static zlib stream allocaions and
deflateReset(), inflateReset() or the like. But I don't
expect a significant difference from that.

It turns out you can get a noticeable win from overriding zlib's
default memory allocator, which calls calloc() even though it has
no need for pre-zeroed storage. Just making it use malloc() instead
saves about 25% of the runtime on 1K-sized strings (the above numbers
include this improvement). We'd probably want to make it use palloc
instead of malloc anyway, so this won't cost us any extra work. I
have not pushed on it to see what else might be gained by tweaking.

What I suggest:

Leave PGLZ in place as the default compressor for toastable
types. Speed is what all benchmarks talk about - on disk
storage size is seldom a minor note.

True, but the other side of the coin is that better compression and
smaller disk space will mean less I/O, better use of shared-memory
disk buffers, etc. So to some extent, better compression helps pay
for itself.

Fix it's history allocation for huge values and have someone
(PgSQL Inc.?) patenting the compression algorithm, so we're
safe at some point in the future.

That would be a really *bad* idea. What will people say if we say
"Postgres contains patented algorithms, but we'll let you use them
for free" ? They'll say "no thanks, I remember Unisys' repeatedly
broken promises about the GIF patent" and stay away in droves.
There is a *lot* of bad blood in the air about compression patents
of any sort. We mustn't risk tainting Postgres' reputation with
that mess.

(In any case, one would hope you couldn't get a patent on this
method, though I suppose it never pays to overestimate the competence
of the USPTO...)

If there's a patent problem
in it, we are already running the risk to get sued, the PGLZ
code got shipped with 7.0, used in lztext.

But it hasn't been documented or advertised. If we take it out
again in 7.1, I think our exposure to potential lawsuits from it is
negligible. Not that I think there is any big risk there anyway,
but we ought to consider the possibility.

My feeling is that going with zlib is probably the right choice.
The technical case for using a homebrew compressor instead isn't
very compelling, and the advantages of using a standardized,
known-patent-free library are not to be ignored.

regards, tom lane

Tom Lane wrote:

JanWieck@t-online.de (Jan Wieck) writes:

Some quick numbers though:
I simply stripped down pg_lzcompress.c to call compress2()
and uncompress() instead of doing anything itself (what a
nice, small source file :-).

I went at it in a different way: pulled out pg_lzcompress into a
standalone source program that could also call zlib. These numbers
represent the pure compression or decompression time for memory-to-
memory processing, no other overhead at all. Each run was iterated
1000 times to make it long enough to time accurately (so you can
read the times as "milliseconds per operation", though they're
really seconds).

We could just make this part extensible as well, like the rest of
postgres, so we would have directory tree like

/compressors
/nullcompressor
/lzcompress
/zlib
/lzo
/bzip2
/my_new_supercompressor
/classic_huffman_for_uppercase_american_english

and select the desired compressor at compilt time, or even better, on
field by field basis at runtime, so that field that stores mainly
tar.gz-s at compression level 9 will use nullcompressor, and others
will use what is best for them.

Fix it's history allocation for huge values and have someone
(PgSQL Inc.?) patenting the compression algorithm, so we're
safe at some point in the future.

That would be a really *bad* idea. What will people say if we say
"Postgres contains patented algorithms, but we'll let you use them
for free" ? They'll say "no thanks, I remember Unisys' repeatedly
broken promises about the GIF patent" and stay away in droves.
There is a *lot* of bad blood in the air about compression patents
of any sort. We mustn't risk tainting Postgres' reputation with
that mess.
(In any case, one would hope you couldn't get a patent on this
method, though I suppose it never pays to overestimate the competence
of the USPTO...)

And AFAIK (IANAL ;) you can only patent previously _unpublished_ work,
even by the patent applicant.

If there's a patent problem
in it, we are already running the risk to get sued, the PGLZ
code got shipped with 7.0, used in lztext.

But it hasn't been documented or advertised. If we take it out
again in 7.1, I think our exposure to potential lawsuits from it is
negligible. Not that I think there is any big risk there anyway,
but we ought to consider the possibility.

My feeling is that going with zlib is probably the right choice.
The technical case for using a homebrew compressor instead isn't
very compelling,

Speed seems to be a good reason, if we can keep it up.

and the advantages of using a standardized,
known-patent-free library are not to be ignored.

OTOH, there are possibly patents on other part of postgres,
like indexing, storage methods, the mere fact that something is
stored in another relation, using 'Z' as a protocol character, etc.
etc. So using a patent-free compression library does not help much.

So if PgSQL Inc. has lots of lawyers with nothing to do, they could do
some patent research and scare all developers away with their findings
;)

-------------
Hannu

I went at it in a different way: pulled out pg_lzcompress into a
standalone source program that could also call zlib.

If you want you could give minilzo a try.

LZO is a data compression library which is suitable for data de-/compression
in real-time.
LZO implements the fastest compression and decompression algorithms around.
See the ratings for lzop in the famous Archive Comparison Test .

LZO needs no memory for decompression, 64k or 8k for compression.

LZO and the LZO algorithms and implementations are distributed under the
terms of the GNU General Public License (GPL) { auf Deutsch }. Special
licenses for commercial and other applications are available by contacting
the author.

See http://wildsau.idv.uni-linz.ac.at/mfx/lzo.html

Andreas

Zeugswetter Andreas SB <ZeugswetterA@wien.spardat.at> writes:

LZO and the LZO algorithms and implementations are distributed under the
terms of the GNU General Public License (GPL) { auf Deutsch }.

GPL is alone a fatal objection for something that has to go into the
guts of Postgres.

More to the point, does it have the same patent-freedom credentials zlib
does? I trust Gailly's opinion about zlib being patent-free because
he spent more time researching the legalities than he did writing code
(and also because zlib has now been around for quite some time with no
challenges). What assurances does LZO offer that we won't be stepping
on a patent mine?

regards, tom lane

LZO and the LZO algorithms and implementations are distributed under the
terms of the GNU General Public License (GPL) { auf Deutsch }.

GPL is alone a fatal objection for something that has to go into the
guts of Postgres.

Yes, but the author states that special projects can get a special license.

What assurances does LZO offer that we won't be stepping on a patent mine?

There was a whole discussion on the author's website explaining what he did
to guarantee the free use of the algorithms, but unfortunately I don't find
it anymore.

The author wants PGP encrypted mail which I don't have.
Maybe he would have a comment for us "BSD License defending PostgreSQL'ers"
if someone forwarded this to him.
(http://wildsau.idv.uni-linz.ac.at/mfx/pgp.html)

I can tell you from experience that the performance of these algorithms is
really
astonishing.

Andreas