Query results caching?

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-
owner@postgresql.org] On Behalf Of Ben-Nes Yonatan
Sent: Monday, August 22, 2005 9:03 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Query results caching?

Hi all,

I dont know how its called but I noticed that when I query the db for
the first time it give me the result slower then the next times ill
repeat the same exact query, I figure that its some kind of caching so
henceforth the title of the mail :)

The operating system and the database will both percolate frequently
used information from disk into memory. Particularly if they are SELECT
queries, they will get faster and faster.

Anyway I would want to be able to delete that "caching" after every
query test that I run, cause I want to see the real time results for

my

queries (its for a searching option for users so it will vary alot).

Those are the real times for your queries.

Is it possible to do it manually each time or maybe only from the
configuration?

You will have to query a different table each time.

Thanks in advance,
Ben-Nes Yonatan
Canaan Surfing ltd.
http://www.canaan.net.il

---------------------------(end of

broadcast)---------------------------

Show quoted text

TIP 2: Don't 'kill -9' the postmaster

On 8/22/05 1:59 PM, "Dann Corbit" <DCorbit@connx.com> wrote:

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-
owner@postgresql.org] On Behalf Of Ben-Nes Yonatan
Sent: Monday, August 22, 2005 9:03 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Query results caching?

Hi all,

I dont know how its called but I noticed that when I query the db for
the first time it give me the result slower then the next times ill
repeat the same exact query, I figure that its some kind of caching so
henceforth the title of the mail :)

The operating system and the database will both percolate frequently
used information from disk into memory. Particularly if they are SELECT
queries, they will get faster and faster.

Anyway I would want to be able to delete that "caching" after every
query test that I run, cause I want to see the real time results for

my

queries (its for a searching option for users so it will vary alot).

Those are the real times for your queries.

Is it possible to do it manually each time or maybe only from the
configuration?

You will have to query a different table each time.

Just to extend this notion a bit, if you want to test your application
speed, you may want to generate "real-world" input to determine the actual
behavior/speed under real conditions. As Dann pointed out, the results for
timings are "real" in that if the user generated the queries as you did, the
timing results would be (nearly) the same as for you. It seems that your
concern is that the user will not generate the same type of input that you
did (that it will vary more), so the best solution may be to actually
generate some test queries that actually conform to what you think the user
input will look like.

Sean

-----Original Message-----
From: Ben-Nes Yonatan [mailto:da@canaan.co.il]
Sent: Monday, August 22, 2005 1:14 PM
To: Sean Davis; Dann Corbit
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Query results caching?

Sean Davis wrote:

On 8/22/05 1:59 PM, "Dann Corbit" <DCorbit@connx.com> wrote:

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-
owner@postgresql.org] On Behalf Of Ben-Nes Yonatan
Sent: Monday, August 22, 2005 9:03 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Query results caching?

Hi all,

I dont know how its called but I noticed that when I query the db

for

the first time it give me the result slower then the next times ill
repeat the same exact query, I figure that its some kind of caching

so

henceforth the title of the mail :)

The operating system and the database will both percolate frequently
used information from disk into memory. Particularly if they are

SELECT

queries, they will get faster and faster.

Anyway I would want to be able to delete that "caching" after every
query test that I run, cause I want to see the real time results

for

my

queries (its for a searching option for users so it will vary

alot).

Those are the real times for your queries.

Is it possible to do it manually each time or maybe only from the
configuration?

You will have to query a different table each time.

Just to extend this notion a bit, if you want to test your

application

speed, you may want to generate "real-world" input to determine the

actual

behavior/speed under real conditions. As Dann pointed out, the

results

for

timings are "real" in that if the user generated the queries as you

did,

the

timing results would be (nearly) the same as for you. It seems that

your

concern is that the user will not generate the same type of input

that

you

did (that it will vary more), so the best solution may be to

actually

generate some test queries that actually conform to what you think

the

user

input will look like.

Sean

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result

(cause

of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table again
with the exact same query as before and now I receive: Total runtime:
216834.871 ms

And for my last check I run the exact same query again (without

creating

the INDEX back again) and I get quite similar result to my third

query:

Total runtime: 209218.01 ms

These results are all what I would expect. When you delete the index,
the query will be forced to do a table scan (to examine every single
record in the table one by one). If the table is non-trivial it is
unlikely that either the OS or the database will cache the whole thing
in memory. However, when you query a small record set, then it is
likely to be retained in RAM which is literally thousands of times
faster than disk.

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about that)
queries were using the "cache" that was created after the first query
(which had an INDEX) so none of them actually showed me what will

happen

if a client will do such a search (without an INDEX) for the first

time.

If a search is to be made on a frequent basis, you should create an
index.
The query results above show you why.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Think about this for a minute. The real life results you want are very
fast results. For that reason, you should try to model the customer
queries as nearly as possible. If you have a canned application like
order entry, then the real parameterized query set will probably be
quite small in real life. If you are creating a server for ad-hoc
queries then it will be far more difficult to model in real life.

What is the real purpose of the application that you are writing?

Will users be using a pre-programmed front end, or will they be typing
in queries free-form for whatever their heart desires?

Show quoted text

Thanks alot again,
Yonatan

On Mon, Aug 22, 2005 at 10:13:49PM +0200, Ben-Nes Yonatan wrote:

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result (cause
of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table again
with the exact same query as before and now I receive: Total runtime:
216834.871 ms

And for my last check I run the exact same query again (without creating
the INDEX back again) and I get quite similar result to my third query:
Total runtime: 209218.01 ms

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about that)
queries were using the "cache" that was created after the first query
(which had an INDEX) so none of them actually showed me what will happen
if a client will do such a search (without an INDEX) for the first time.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Emptying the cache will not show real-life results. You are always going
to have some stuff cached, even if you get a query for something new. In
this case (since you'll obviously want those indexes there), after some
amount of time you will have most (if not all) of the non-leaf index
pages cached, since they take a fairly small amount of memory and are
frequently accessed. This makes index traversal *much* faster than your
initial case shows, even if you query on something different each time.
Testing with a completely empty cache just isn't that realistic.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com 512-569-9461

Sean Davis wrote:

On 8/22/05 1:59 PM, "Dann Corbit" <DCorbit@connx.com> wrote:

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-
owner@postgresql.org] On Behalf Of Ben-Nes Yonatan
Sent: Monday, August 22, 2005 9:03 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] Query results caching?

Hi all,

I dont know how its called but I noticed that when I query the db for
the first time it give me the result slower then the next times ill
repeat the same exact query, I figure that its some kind of caching so
henceforth the title of the mail :)

The operating system and the database will both percolate frequently
used information from disk into memory. Particularly if they are SELECT
queries, they will get faster and faster.

Anyway I would want to be able to delete that "caching" after every
query test that I run, cause I want to see the real time results for

my

queries (its for a searching option for users so it will vary alot).

Those are the real times for your queries.

Is it possible to do it manually each time or maybe only from the
configuration?

You will have to query a different table each time.

Just to extend this notion a bit, if you want to test your application
speed, you may want to generate "real-world" input to determine the actual
behavior/speed under real conditions. As Dann pointed out, the results for
timings are "real" in that if the user generated the queries as you did, the
timing results would be (nearly) the same as for you. It seems that your
concern is that the user will not generate the same type of input that you
did (that it will vary more), so the best solution may be to actually
generate some test queries that actually conform to what you think the user
input will look like.

Sean

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result (cause
of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table again
with the exact same query as before and now I receive: Total runtime:
216834.871 ms

And for my last check I run the exact same query again (without creating
the INDEX back again) and I get quite similar result to my third query:
Total runtime: 209218.01 ms

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about that)
queries were using the "cache" that was created after the first query
(which had an INDEX) so none of them actually showed me what will happen
if a client will do such a search (without an INDEX) for the first time.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Thanks alot again,
Yonatan

am 22.08.2005, um 22:13:49 +0200 mailte Ben-Nes Yonatan folgendes:

I think that I was misunderstood, Ill make an example:

Okay:

Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
.. Total runtime: 3139.587 ms

neither the DB nor the OS has the the table and index in the cache.

Now im doing the same query again and i get a much faster result (cause of
the "caching"): Total runtime: 332.53 ms

OS and DN has now the table and index in the cache.

After both of those queries I drop the index and query the table again with
the exact same query as before and now I receive: Total runtime: 216834.871
ms

Without index -> DB make a seq-scan. Very slow, of cource.

And for my last check I run the exact same query again (without creating
the INDEX back again) and I get quite similar result to my third query:
Total runtime: 209218.01 ms

Never mind. The table is too big for the cache.

My problem is that (maybe I just dont understand something basic here...)
the last 2 (also the second query but I dont care about that) queries were
using the "cache" that was created after the first query (which had an
INDEX) so none of them actually showed me what will happen if a client will
do such a search (without an INDEX) for the first time.

I want to delete that "caching" after I do the first 2 queries so my next
queries will show me "real life results".

No problem: demount all RAM and send this to me ;-)

Regards, Andreas
--
Andreas Kretschmer (Kontakt: siehe Header)
Heynitz: 035242/47212, D1: 0160/7141639
GnuPG-ID 0x3FFF606C http://wwwkeys.de.pgp.net
=== Schollglas Unternehmensgruppe ===

-----Original Message-----
From: Ben-Nes Yonatan [mailto:da@canaan.co.il]
Sent: Monday, August 22, 2005 3:28 PM
To: Jim C. Nasby; Sean Davis; Dann Corbit
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Query results caching?

On Mon, Aug 22, 2005 at 10:13:49PM +0200, Ben-Nes Yonatan wrote:

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on

the

"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result

(cause

of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table

again

with the exact same query as before and now I receive: Total

runtime:

216834.871 ms

And for my last check I run the exact same query again (without

creating

the INDEX back again) and I get quite similar result to my third

query:

Total runtime: 209218.01 ms

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about

that)

queries were using the "cache" that was created after the first

query

(which had an INDEX) so none of them actually showed me what will

happen

if a client will do such a search (without an INDEX) for the first

time.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Ok I tried to handle both of your replies cause I got them at 2

seperate

emails.

Dann Corbit wrote:

These results are all what I would expect. When you delete the

index,

the query will be forced to do a table scan (to examine every

single

record in the table one by one). If the table is non-trivial it is
unlikely that either the OS or the database will cache the whole

thing

in memory. However, when you query a small record set, then it is
likely to be retained in RAM which is literally thousands of times
faster than disk.

Didnt know that, good to know though doesnt assure me...
What if I drop the INDEX but create a diffrent INDEX which also make

the

process alot faster then without an INDEX but slower/faster then the

one

before, will it wont use the former "caching"?

You can add several indexes to a single table.
If you do some statistics on the query patterns, you can find what
indexes are needed to make the queries as fast as possible.

If a search is to be made on a frequent basis, you should create an
index.
The query results above show you why.

Obvious :)

Think about this for a minute. The real life results you want are
very fast results. For that reason, you should try to model the
customer queries as nearly as possible. If you have a canned
application like order entry, then the real parameterized query set
will probably be quite small in real life. If you are creating a
server for ad-hoc queries then it will be far more difficult to

model

in real life.

What is the real purpose of the application that you are writing?

Will users be using a pre-programmed front end, or will they be

typing

in queries free-form for whatever their heart desires?

Ok ill try to describe the system as short & precise as possible (its
also passed midnight here :)).
Each day I receive about 4 million rows of data (products) which I
insert into table1 (after I delete all of the previous data it had),
along it I receive for every row about another 15 keywords which I
insert into table2 (where as in table1 I delete all of the previous

data

it had also), this process is a fact that I cant change.

If the data arrives on a daily basis, and is not updated until the next
day, I suggest creating a lot of indexes, and cluster on the index used
most frequently.

What exactly are the 15 keywords in the second table for?
Are they column names?
Are they categories for the first table?
Why is the second table necessary at all?

Now the users of the site can search for data from table1 by typing
whichever (and up to 4) words as they want at a text field (search

input

string) and the server should display the correct results by querying
table1 & join table2 for its keywords.

Can you give the exact table definitions for the two tables, and also
the most likely queries you are going to receive?

When the users type in keywords -- can these keywords be applied against
any column in the table or only against a single column or against a
small set of columns or something else?

I succeded to do it quite fast but when I tried to ORDER BY my results
its times jumped up drastically (2-3 seconds for a query... and thats
after the caching..).

Order by will complicate quite a bit. I have not tried it on
PostgreSQL, but if you know the result set is small, a technique is to
select into a temp table and then order by on the temp table. It works
well on other database systems (caveat: it has been a while since I
worked as a DBA and I have not worked as a DBA on PostgreSQL).

I can't allow a situation where a user will search with a keyword

which

wasnt 'cached' before and because of that he will wait 15 seconds for

a

result.

You might try throwing hardware at it. A 4 CPU AMD 64 machine with
Ultra 320 striped SCSI disk array and a few gigabytes of ram will
perform admirably.

You might want more than is possible. If you have 4 million rows, and
each row is 1K, then that is 4 GB. If your users do a query that has
not been performed yet and you have to do a table scan, then you cannot
expect some kind of sub-second response times because it won't be
physically possible on any system.

If you know what most queries may look like or if you only have a few
character columns so that you can make an index on each of them and if
you can put a unique clustered index on the most important (frequently
used) item, then you can get the majority of your queries to run very
quickly, and only on rare occasions will the query be slow.

If I have a 4 million row table, with long rows and big varchar columns
and I run a query on a column like this:

SELECT * FROM inventory WHERE product LIKE '%Table%'

It isn't going to be fast on any system with any database.
[snip]

On Mon, Aug 22, 2005 at 10:13:49PM +0200, Ben-Nes Yonatan wrote:

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result (cause
of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table again
with the exact same query as before and now I receive: Total runtime:
216834.871 ms

And for my last check I run the exact same query again (without creating
the INDEX back again) and I get quite similar result to my third query:
Total runtime: 209218.01 ms

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about that)
queries were using the "cache" that was created after the first query
(which had an INDEX) so none of them actually showed me what will happen
if a client will do such a search (without an INDEX) for the first time.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Ok I tried to handle both of your replies cause I got them at 2 seperate
emails.

Dann Corbit wrote:

These results are all what I would expect. When you delete the index,
the query will be forced to do a table scan (to examine every single
record in the table one by one). If the table is non-trivial it is
unlikely that either the OS or the database will cache the whole thing
in memory. However, when you query a small record set, then it is
likely to be retained in RAM which is literally thousands of times
faster than disk.

Didnt know that, good to know though doesnt assure me...
What if I drop the INDEX but create a diffrent INDEX which also make the
process alot faster then without an INDEX but slower/faster then the one
before, will it wont use the former "caching"?

If a search is to be made on a frequent basis, you should create an
index.
The query results above show you why.

Obvious :)

Think about this for a minute. The real life results you want are
very fast results. For that reason, you should try to model the
customer queries as nearly as possible. If you have a canned
application like order entry, then the real parameterized query set
will probably be quite small in real life. If you are creating a
server for ad-hoc queries then it will be far more difficult to model
in real life.

What is the real purpose of the application that you are writing?

Will users be using a pre-programmed front end, or will they be typing
in queries free-form for whatever their heart desires?

Ok ill try to describe the system as short & precise as possible (its
also passed midnight here :)).
Each day I receive about 4 million rows of data (products) which I
insert into table1 (after I delete all of the previous data it had),
along it I receive for every row about another 15 keywords which I
insert into table2 (where as in table1 I delete all of the previous data
it had also), this process is a fact that I cant change.
Now the users of the site can search for data from table1 by typing
whichever (and up to 4) words as they want at a text field (search input
string) and the server should display the correct results by querying
table1 & join table2 for its keywords.
I succeded to do it quite fast but when I tried to ORDER BY my results
its times jumped up drastically (2-3 seconds for a query... and thats
after the caching..).
I can't allow a situation where a user will search with a keyword which
wasnt 'cached' before and because of that he will wait 15 seconds for a
result.

Jim C. Nasby wrote:

Emptying the cache will not show real-life results. You are always going
to have some stuff cached, even if you get a query for something new. In
this case (since you'll obviously want those indexes there), after some
amount of time you will have most (if not all) of the non-leaf index
pages cached, since they take a fairly small amount of memory and are
frequently accessed. This makes index traversal *much* faster than your
initial case shows, even if you query on something different each time.
Testing with a completely empty cache just isn't that realistic.

As far as I understand it at my situation where all of the data is
deleted and inserted each day from the start (INDEX will get lost with
it..) & the endless variety of possible keywords search's & the immense
size of the tables, the following reason wont last.. or am I wrong here?

Because of all of that I want to be able to see how much time a query
takes when its the first time its being run..... or I'm wrong again and
failing to understand something?

Again everyone THANKS ALOT its really amazing the help that I receive
from you!
Ben-Nes Yonatan

am 22.08.2005, um 22:13:49 +0200 mailte Ben-Nes Yonatan folgendes:

I think that I was misunderstood, Ill make an example:

Okay:

Lets say that im making the following query for the first time on the
"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
.. Total runtime: 3139.587 ms

neither the DB nor the OS has the the table and index in the cache.

Now im doing the same query again and i get a much faster result (cause of
the "caching"): Total runtime: 332.53 ms

OS and DN has now the table and index in the cache.

After both of those queries I drop the index and query the table again with
the exact same query as before and now I receive: Total runtime: 216834.871
ms

Without index -> DB make a seq-scan. Very slow, of cource.

And for my last check I run the exact same query again (without creating
the INDEX back again) and I get quite similar result to my third query:
Total runtime: 209218.01 ms

Never mind. The table is too big for the cache.

My problem is that (maybe I just dont understand something basic here...)
the last 2 (also the second query but I dont care about that) queries were
using the "cache" that was created after the first query (which had an
INDEX) so none of them actually showed me what will happen if a client will
do such a search (without an INDEX) for the first time.

I want to delete that "caching" after I do the first 2 queries so my next
queries will show me "real life results".

No problem: demount all RAM and send this to me ;-)

Regards, Andreas

heheheh sure to which address should I send it? :P

On Tue, Aug 23, 2005 at 12:27:39AM +0200, Ben-Nes Yonatan wrote:

Jim C. Nasby wrote:

Emptying the cache will not show real-life results. You are always going
to have some stuff cached, even if you get a query for something new. In
this case (since you'll obviously want those indexes there), after some
amount of time you will have most (if not all) of the non-leaf index
pages cached, since they take a fairly small amount of memory and are
frequently accessed. This makes index traversal *much* faster than your
initial case shows, even if you query on something different each time.
Testing with a completely empty cache just isn't that realistic.

As far as I understand it at my situation where all of the data is
deleted and inserted each day from the start (INDEX will get lost with
it..) & the endless variety of possible keywords search's & the immense
size of the tables, the following reason wont last.. or am I wrong here?

You're wrong - to an extent. Remember that while you're loading all that
data it's also being cached. Now, some of it will probably end up
falling out of the cache as all the data is read in, but you certainly
won't be starting from the clean slate that you're looking for.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com 512-569-9461

Jim C. Nasby wrote:

On Tue, Aug 23, 2005 at 12:27:39AM +0200, Ben-Nes Yonatan wrote:

Jim C. Nasby wrote:

Emptying the cache will not show real-life results. You are always going
to have some stuff cached, even if you get a query for something new. In
this case (since you'll obviously want those indexes there), after some
amount of time you will have most (if not all) of the non-leaf index
pages cached, since they take a fairly small amount of memory and are
frequently accessed. This makes index traversal *much* faster than your
initial case shows, even if you query on something different each time.
Testing with a completely empty cache just isn't that realistic.

As far as I understand it at my situation where all of the data is
deleted and inserted each day from the start (INDEX will get lost with
it..) & the endless variety of possible keywords search's & the immense
size of the tables, the following reason wont last.. or am I wrong here?

You're wrong - to an extent. Remember that while you're loading all that
data it's also being cached. Now, some of it will probably end up
falling out of the cache as all the data is read in, but you certainly
won't be starting from the clean slate that you're looking for.

Ok I guess that if all of you are telling me this over and over then it
probably got some point in it :), I guess that I'll just see it work by
time.

Thanks alot again (I really appreciate it),
Ben-Nes Yonatan
Canaan Surfing ltd.
http://www.canaan.net.il

Dann Corbit wrote:

-----Original Message-----
From: Ben-Nes Yonatan [mailto:da@canaan.co.il]
Sent: Monday, August 22, 2005 3:28 PM
To: Jim C. Nasby; Sean Davis; Dann Corbit
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Query results caching?

On Mon, Aug 22, 2005 at 10:13:49PM +0200, Ben-Nes Yonatan wrote:

I think that I was misunderstood, Ill make an example:
Lets say that im making the following query for the first time on

the

"motorcycles" table which got an index on the "manufacturer" field:

EXPLAIN ANALYZE SELECT manufacturer FROM motorcycles WHERE
manufacturer='suzuki';
... Total runtime: 3139.587 ms

Now im doing the same query again and i get a much faster result

(cause

of the "caching"): Total runtime: 332.53 ms

After both of those queries I drop the index and query the table

again

with the exact same query as before and now I receive: Total

runtime:

216834.871 ms

And for my last check I run the exact same query again (without

creating

the INDEX back again) and I get quite similar result to my third

query:

Total runtime: 209218.01 ms

My problem is that (maybe I just dont understand something basic
here...) the last 2 (also the second query but I dont care about

that)

queries were using the "cache" that was created after the first

query

(which had an INDEX) so none of them actually showed me what will

happen

if a client will do such a search (without an INDEX) for the first

time.

I want to delete that "caching" after I do the first 2 queries so my
next queries will show me "real life results".

Ok I tried to handle both of your replies cause I got them at 2

seperate

emails.

Dann Corbit wrote:

These results are all what I would expect. When you delete the

index,

the query will be forced to do a table scan (to examine every

single

record in the table one by one). If the table is non-trivial it is
unlikely that either the OS or the database will cache the whole

thing

in memory. However, when you query a small record set, then it is
likely to be retained in RAM which is literally thousands of times
faster than disk.

Didnt know that, good to know though doesnt assure me...
What if I drop the INDEX but create a diffrent INDEX which also make

the

process alot faster then without an INDEX but slower/faster then the

one

before, will it wont use the former "caching"?

You can add several indexes to a single table.
If you do some statistics on the query patterns, you can find what
indexes are needed to make the queries as fast as possible.

If a search is to be made on a frequent basis, you should create an
index.
The query results above show you why.

Obvious :)

Think about this for a minute. The real life results you want are
very fast results. For that reason, you should try to model the
customer queries as nearly as possible. If you have a canned
application like order entry, then the real parameterized query set
will probably be quite small in real life. If you are creating a
server for ad-hoc queries then it will be far more difficult to

model

in real life.

What is the real purpose of the application that you are writing?

Will users be using a pre-programmed front end, or will they be

typing

in queries free-form for whatever their heart desires?

Ok ill try to describe the system as short & precise as possible (its
also passed midnight here :)).
Each day I receive about 4 million rows of data (products) which I
insert into table1 (after I delete all of the previous data it had),
along it I receive for every row about another 15 keywords which I
insert into table2 (where as in table1 I delete all of the previous

data

it had also), this process is a fact that I cant change.

If the data arrives on a daily basis, and is not updated until the next
day, I suggest creating a lot of indexes, and cluster on the index used
most frequently.

What exactly are the 15 keywords in the second table for?
Are they column names?
Are they categories for the first table?
Why is the second table necessary at all?

Now clustering was unknown to me when I received this email from you...
THANKS!!! I created 4 replicas of my table ordered by the diffrent order
that I want to allow my users to use, yep its quite alot of GB but I
dont care about it as long its working fast, and damn its flying! less
then 100 ms and thats on a weak server which will be replaced soon!.

Now the users of the site can search for data from table1 by typing
whichever (and up to 4) words as they want at a text field (search

input

string) and the server should display the correct results by querying
table1 & join table2 for its keywords.

Can you give the exact table definitions for the two tables, and also
the most likely queries you are going to receive?

When the users type in keywords -- can these keywords be applied against
any column in the table or only against a single column or against a
small set of columns or something else?

Well yea its working on a single column and that column got indexed so
its flying.

I succeded to do it quite fast but when I tried to ORDER BY my results
its times jumped up drastically (2-3 seconds for a query... and thats
after the caching..).

Order by will complicate quite a bit. I have not tried it on
PostgreSQL, but if you know the result set is small, a technique is to
select into a temp table and then order by on the temp table. It works
well on other database systems (caveat: it has been a while since I
worked as a DBA and I have not worked as a DBA on PostgreSQL).

That cant work cause it wont order all of the results by the desired
column but only the returned results so when a user will want to see
more results he can see results which were supposed to be displayed
before his previous display.

I can't allow a situation where a user will search with a keyword

which

wasnt 'cached' before and because of that he will wait 15 seconds for

a

result.

You might try throwing hardware at it. A 4 CPU AMD 64 machine with
Ultra 320 striped SCSI disk array and a few gigabytes of ram will
perform admirably.

You might want more than is possible. If you have 4 million rows, and
each row is 1K, then that is 4 GB. If your users do a query that has
not been performed yet and you have to do a table scan, then you cannot
expect some kind of sub-second response times because it won't be
physically possible on any system.

If you know what most queries may look like or if you only have a few
character columns so that you can make an index on each of them and if
you can put a unique clustered index on the most important (frequently
used) item, then you can get the majority of your queries to run very
quickly, and only on rare occasions will the query be slow.

If I have a 4 million row table, with long rows and big varchar columns
and I run a query on a column like this:

SELECT * FROM inventory WHERE product LIKE '%Table%'

It isn't going to be fast on any system with any database.
[snip]

Well a better hardware will soon be working but anyway your idea about
clustering solved my problem.

Thanks alot again! :)
Ben-Nes Yonatan
Canaan Surfing ltd.
http://www.canaan.net.il

Dann Corbit wrote:

If I have a 4 million row table, with long rows and big varchar columns
and I run a query on a column like this:

SELECT * FROM inventory WHERE product LIKE '%Table%'

It isn't going to be fast on any system with any database.

Hypothetically it seems one could theoretically use some sort
of GIST index not unlike the contrib/trigram stuff to speed
up like clauses like that. If so, I wonder if down the road
that could be a nice competitive advantage over systems with
less flexible index systems. Is that a possible TODO?