Creating table with data from a join

On 14 July 2015 at 21:12, Igor Stassiy <istassiy@gmail.com> wrote:

Hello,

I am benchmarking different ways of putting data into table on table
creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id = b.id;
2. CREATE TABLE c AS SELECT * FROM a JOIN b on a.id = b.id;
3. psql -c "COPY (SELECT * FROM a JOIN b on a.id = b.id) TO STDOUT" |
parallel --block 128M --jobs 4 --pipe psql -c "COPY c FROM STDIN";

(the parallel command is available as part of parallel deb package in
Ubuntu for example, it splits the stdin by newline character and feeds it
to the corresponding command)

Both tables a and b have ~16M records and one of the columns in a is
geometry (ranging from several KB in size to several MB). Columns in b are
mostly integers.

The machine that I am running these commands on has the following
parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06
maintenance_work_mem = 1GB # pgtune wizard 2012-06-06 constraint_exclusion
= on # pgtune wizard 2012-06-06 checkpoint_completion_target = 0.9 # pgtune
wizard 2012-06-06 effective_cache_size = 48GB # pgtune wizard 2012-06-06
work_mem = 80MB # pgtune wizard 2012-06-06
wal_buffers = 8MB # pgtune wizard 2012-06-06
checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06
max_connections = 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2, however 2
outperforms both by a large margin (sometimes x2). This is especially
surprising taking into account that COPY doesn't acquire a global lock on
the table, only a RowExclusiveLock
(according to
/messages/by-id/10611.1014867684@sss.pgh.pa.us)

So is option 2 a winner by design? Could you please suggest other
alternatives to try (if there are any)? And what might be the reason that 3
is not outperforming the other 2?

I would imagine that the calling of the output function to translate the
each value's internal representation to it's user visible/external
representation plus all the overhead of sending results to the client would
be a likely candidate of the slow down. In either case 3 would only be as
fast as the query generating the output. With 1 and 2 all the tuple
representations of each record stays in the internal format.

If you have some logical way to break the query down into parts, then maybe
that would be a place to look.
For example:

INSERT INTO c SELECT * FROM a JOIN b on a.id = b.id WHERE a.id < 8000000;
INSERT INTO c SELECT * FROM a JOIN b on a.id = b.id WHERE a.id >= 8000000;

Of course, you'd need to be very careful to ensure that the results of each
SELECT never overlap. It would be nice to invent some better way than this
that divided the workload evenly even when the tables grow.

Then you could run these concurrently.

Regards

David Rowley

--
David Rowley http://www.2ndQuadrant.com/
<http://www.2ndquadrant.com/&gt;
PostgreSQL Development, 24x7 Support, Training & Services

Thank you David. I tried to measure the time of COPY (SELECT * FROM a JOIN
b ON a.id = b.id) TO '/tmp/dump.sql' and it took an order of magnitude time
less (~10x) than the complete command (together with INSERT), so conversion
is probably not the main factor of slowdown (unless conversion from text
->internal is significantly slower than that of from internal -> text).

I will also try your suggestion with limiting the ids range.

On Tue, Jul 14, 2015 at 1:42 PM David Rowley <david.rowley@2ndquadrant.com>
wrote:

On 14/07/2015 11:12, Igor Stassiy wrote:

Hello,

I am benchmarking different ways of putting data into table on table
creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id <http://a.id&gt; = b.id
<http://b.id&gt;;
2. CREATE TABLE c AS SELECT * FROM a JOIN b on a.id <http://a.id&gt; = b.id
<http://b.id&gt;;
3. psql -c "COPY (SELECT * FROM a JOIN b on a.id <http://a.id&gt; = b.id
<http://b.id&gt;) TO STDOUT" |
parallel --block 128M --jobs 4 --pipe psql -c "COPY c FROM STDIN";

(the parallel command is available as part of parallel deb package in
Ubuntu for example, it splits the stdin by newline character and feeds
it to the corresponding command)

Both tables a and b have ~16M records and one of the columns in a is
geometry (ranging from several KB in size to several MB). Columns in b
are mostly integers.

The machine that I am running these commands on has the following
parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06
maintenance_work_mem = 1GB # pgtune wizard 2012-06-06
constraint_exclusion = on # pgtune wizard 2012-06-06
checkpoint_completion_target = 0.9 # pgtune wizard 2012-06-06
effective_cache_size = 48GB # pgtune wizard 2012-06-06
work_mem = 80MB # pgtune wizard 2012-06-06
wal_buffers = 8MB # pgtune wizard 2012-06-06
checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06
max_connections = 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2, however 2
outperforms both by a large margin (sometimes x2). This is especially
surprising taking into account that COPY doesn't acquire a global lock
on the table, only a RowExclusiveLock
(according
to /messages/by-id/10611.1014867684@sss.pgh.pa.us)

What is wal_level value? I think this is because of an optimisation
happening with wal_level = minimal:

"In minimal level, WAL-logging of some bulk operations can be safely
skipped, which can make those operations much faster"

see http://www.postgresql.org/docs/current/static/runtime-config-wal.html

So is option 2 a winner by design? Could you please suggest other
alternatives to try (if there are any)? And what might be the reason
that 3 is not outperforming the other 2?

Thank you,
Igor

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org

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Julien, I have the following setting for WAL level: #wal_level = minimal
(which defaults to minimal anyway)

On Tue, Jul 14, 2015 at 6:19 PM Julien Rouhaud <julien.rouhaud@dalibo.com>
wrote:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

On 14/07/2015 18:21, Igor Stassiy wrote:

Julien, I have the following setting for WAL level: #wal_level =
minimal (which defaults to minimal anyway)

On Tue, Jul 14, 2015 at 6:19 PM Julien Rouhaud
<julien.rouhaud@dalibo.com <mailto:julien.rouhaud@dalibo.com>>
wrote:

On 14/07/2015 11:12, Igor Stassiy wrote:

Hello,

I am benchmarking different ways of putting data into table on
table creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;; 2. CREATE TABLE c AS SELECT * FROM a JOIN b on
a.id <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;; 3. psql -c "COPY (SELECT * FROM a JOIN b on a.id
<http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;) TO STDOUT" | parallel --block 128M --jobs 4 --pipe
psql -c "COPY c FROM STDIN";

(the parallel command is available as part of parallel deb
package in Ubuntu for example, it splits the stdin by newline
character and feeds it to the corresponding command)

Both tables a and b have ~16M records and one of the columns in a
is geometry (ranging from several KB in size to several MB).
Columns in b are mostly integers.

The machine that I am running these commands on has the
following parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06
maintenance_work_mem = 1GB # pgtune wizard 2012-06-06
constraint_exclusion = on # pgtune wizard 2012-06-06
checkpoint_completion_target = 0.9 # pgtune wizard 2012-06-06
effective_cache_size = 48GB # pgtune wizard 2012-06-06 work_mem =
80MB # pgtune wizard 2012-06-06 wal_buffers = 8MB # pgtune wizard
2012-06-06 checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06 max_connections
= 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2,
however 2 outperforms both by a large margin (sometimes x2). This
is especially surprising taking into account that COPY doesn't
acquire a global lock on the table, only a RowExclusiveLock
(according to

/messages/by-id/10611.1014867684@sss.pgh.pa.us)

What is wal_level value? I think this is because of an
optimisation happening with wal_level = minimal:

"In minimal level, WAL-logging of some bulk operations can be
safely skipped, which can make those operations much faster"

see
http://www.postgresql.org/docs/current/static/runtime-config-wal.html

So is option 2 a winner by design? Could you please suggest
other alternatives to try (if there are any)? And what might be
the reason that 3 is not outperforming the other 2?

Thank you, Igor

-- Julien Rouhaud http://dalibo.com - http://dalibo.org

- --
Julien Rouhaud
http://dalibo.com - http://dalibo.org
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On 14/07/2015 18:21, Igor Stassiy wrote:

Julien, I have the following setting for WAL level: #wal_level = minimal
(which defaults to minimal anyway)

Sorry, I sent my mail too early :/

So, option #2 is winner by design. You didn't say anything about your
needs, so it's hard to help you much more.

If you don't care about losing data on this table if your server
crashes, you can try option #3 with an unlogged table.

On Tue, Jul 14, 2015 at 6:19 PM Julien Rouhaud
<julien.rouhaud@dalibo.com <mailto:julien.rouhaud@dalibo.com>> wrote:

On 14/07/2015 11:12, Igor Stassiy wrote:

Hello,

I am benchmarking different ways of putting data into table on table
creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;;
2. CREATE TABLE c AS SELECT * FROM a JOIN b on a.id <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;;
3. psql -c "COPY (SELECT * FROM a JOIN b on a.id <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt;

<http://b.id&gt;) TO STDOUT" |
parallel --block 128M --jobs 4 --pipe psql -c "COPY c FROM STDIN";

(the parallel command is available as part of parallel deb package in
Ubuntu for example, it splits the stdin by newline character and feeds
it to the corresponding command)

Both tables a and b have ~16M records and one of the columns in a is
geometry (ranging from several KB in size to several MB). Columns in b
are mostly integers.

The machine that I am running these commands on has the following
parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06
maintenance_work_mem = 1GB # pgtune wizard 2012-06-06
constraint_exclusion = on # pgtune wizard 2012-06-06
checkpoint_completion_target = 0.9 # pgtune wizard 2012-06-06
effective_cache_size = 48GB # pgtune wizard 2012-06-06
work_mem = 80MB # pgtune wizard 2012-06-06
wal_buffers = 8MB # pgtune wizard 2012-06-06
checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06
max_connections = 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2, however 2
outperforms both by a large margin (sometimes x2). This is especially
surprising taking into account that COPY doesn't acquire a global lock
on the table, only a RowExclusiveLock
(according
to

/messages/by-id/10611.1014867684@sss.pgh.pa.us)

What is wal_level value? I think this is because of an optimisation
happening with wal_level = minimal:

"In minimal level, WAL-logging of some bulk operations can be safely
skipped, which can make those operations much faster"

see
http://www.postgresql.org/docs/current/static/runtime-config-wal.html

So is option 2 a winner by design? Could you please suggest other
alternatives to try (if there are any)? And what might be the reason
that 3 is not outperforming the other 2?

Thank you,
Igor

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org

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Julien, I would gladly provide more information, I am just not sure what to
add.

I would be willing to leave the server compromised for things like corrupts
or data losses during the time of this import, but the server has to be up
and running before and after the import, if it is successful (so I can't
take it down then change some parameters and start it up with again).

On Tue, Jul 14, 2015 at 6:37 PM Julien Rouhaud <julien.rouhaud@dalibo.com>
wrote:

Hello,

I am benchmarking different ways of putting data into table on table creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id = b.id;
2. CREATE TABLE c AS SELECT * FROM a JOIN b on a.id = b.id;
3. psql -c "COPY (SELECT * FROM a JOIN b on a.id = b.id) TO STDOUT" |
parallel --block 128M --jobs 4 --pipe psql -c "COPY c FROM STDIN";

(the parallel command is available as part of parallel deb package in Ubuntu for example, it splits the stdin by newline character and feeds it to the corresponding command)

Both tables a and b have ~16M records and one of the columns in a is geometry (ranging from several KB in size to several MB). Columns in b are mostly integers.

The machine that I am running these commands on has the following parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06 maintenance_work_mem = 1GB # pgtune wizard 2012-06-06 constraint_exclusion = on # pgtune wizard 2012-06-06 checkpoint_completion_target = 0.9 # pgtune wizard 2012-06-06 effective_cache_size = 48GB # pgtune wizard 2012-06-06
work_mem = 80MB # pgtune wizard 2012-06-06
wal_buffers = 8MB # pgtune wizard 2012-06-06
checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06
max_connections = 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2, however 2 outperforms both by a large margin (sometimes x2). This is especially surprising taking into account that COPY doesn't acquire a global lock on the table, only a RowExclusiveLock

which PG Version ?

I find interesting, that 2 outperforms 1.
The only explanation I can imagine is that "CREATE TABLE AS" freezes the data on the fly, as possible with "COPY FROM"
(http://www.postgresql.org/docs/9.4/interactive/sql-copy.html)

You may try parallel insert without using STDIN using modulo. Just start these 4 queries simultaneously:
INSERT INTO c SELECT * FROM a JOIN b on a.id = b.id
WHERE a.id%4 = [0,1,2,3}

I usually avoid parallel INSERTS to avoid I/O contention and random distribution within the target tables.
Are you monitoring the I/O activity in your tests ?
Have you tried to use only 2 parallel processes?

regards,

Marc Mamin

On 14/07/2015 18:50, Igor Stassiy wrote:

Julien, I would gladly provide more information, I am just not sure what
to add.

Well, was your concern about why option #2 is the quickest, or is this
runtime with option #2 still too slow for you ?

I would be willing to leave the server compromised for things like
corrupts or data losses during the time of this import, but the server
has to be up and running before and after the import, if it is
successful (so I can't take it down then change some parameters and
start it up with again).

Check http://www.postgresql.org/docs/current/static/sql-createtable.html
and the "UNLOGGED" part to check if an unlogged table is suitable for you.

On Tue, Jul 14, 2015 at 6:37 PM Julien Rouhaud
<julien.rouhaud@dalibo.com <mailto:julien.rouhaud@dalibo.com>> wrote:

On 14/07/2015 18:21, Igor Stassiy wrote:

Julien, I have the following setting for WAL level: #wal_level =

minimal

(which defaults to minimal anyway)

Sorry, I sent my mail too early :/

So, option #2 is winner by design. You didn't say anything about your
needs, so it's hard to help you much more.

If you don't care about losing data on this table if your server
crashes, you can try option #3 with an unlogged table.

On Tue, Jul 14, 2015 at 6:19 PM Julien Rouhaud
<julien.rouhaud@dalibo.com <mailto:julien.rouhaud@dalibo.com>

<mailto:julien.rouhaud@dalibo.com
<mailto:julien.rouhaud@dalibo.com>>> wrote:

On 14/07/2015 11:12, Igor Stassiy wrote:

Hello,

I am benchmarking different ways of putting data into table

on table

creation:

1. INSERT INTO c SELECT * FROM a JOIN b on a.id

<http://a.id&gt; <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt; <http://b.id&gt;

<http://b.id&gt;;
2. CREATE TABLE c AS SELECT * FROM a JOIN b on a.id

<http://a.id&gt; <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt; <http://b.id&gt;

<http://b.id&gt;;
3. psql -c "COPY (SELECT * FROM a JOIN b on a.id

<http://a.id&gt; <http://a.id&gt;

<http://a.id&gt; = b.id <http://b.id&gt; <http://b.id&gt;

<http://b.id&gt;) TO STDOUT" |
parallel --block 128M --jobs 4 --pipe psql -c "COPY c FROM

STDIN";

(the parallel command is available as part of parallel deb

package in

Ubuntu for example, it splits the stdin by newline character

and feeds

it to the corresponding command)

Both tables a and b have ~16M records and one of the columns

in a is

geometry (ranging from several KB in size to several MB).

Columns in b

are mostly integers.

The machine that I am running these commands on has the

following

parameters:

default_statistics_target = 50 # pgtune wizard 2012-06-06
maintenance_work_mem = 1GB # pgtune wizard 2012-06-06
constraint_exclusion = on # pgtune wizard 2012-06-06
checkpoint_completion_target = 0.9 # pgtune wizard 2012-06-06
effective_cache_size = 48GB # pgtune wizard 2012-06-06
work_mem = 80MB # pgtune wizard 2012-06-06
wal_buffers = 8MB # pgtune wizard 2012-06-06
checkpoint_segments = 16 # pgtune wizard 2012-06-06
shared_buffers = 16GB # pgtune wizard 2012-06-06
max_connections = 400 # pgtune wizard 2012-06-06

One would expect the 3rd option to be faster than 1 and 2,

however 2

outperforms both by a large margin (sometimes x2). This is

especially

surprising taking into account that COPY doesn't acquire a

global lock

on the table, only a RowExclusiveLock
(according
to

/messages/by-id/10611.1014867684@sss.pgh.pa.us)

What is wal_level value? I think this is because of an

optimisation

happening with wal_level = minimal:

"In minimal level, WAL-logging of some bulk operations can be

safely

skipped, which can make those operations much faster"

see

http://www.postgresql.org/docs/current/static/runtime-config-wal.html

So is option 2 a winner by design? Could you please suggest

other

alternatives to try (if there are any)? And what might be

the reason

that 3 is not outperforming the other 2?

Thank you,
Igor

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org

--
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http://dalibo.com - http://dalibo.org

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David, I did something like this:

psql -f /dev/fd/3 3 << IN1 & psql -f /dev/fd/4 4 << IN2 ...
INSERT INTO c SELECT * FRO a JOIN b ON a.ad=b.id WHERE a.id < 0.25th
quantile
IN1
INSERT INTO c SELECT * FRO a JOIN b ON a.ad=b.id WHERE a.id < 0.5th
quantile AND a.id >= 0.25th quantile
IN2
...
IN3
...
IN4

And quantiles were computed using the function:

SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY id) FROM a, and then
once more for lower and upper halves. But unfortunately, I got only about
16% improvement from non-parallelized version of INSERT INTO .. SELECT ..

Marc, I am using postgres 9.4. I didn't benchmark, but intuitively the
modulo operator will force traversing every record in table "a" 4 times, as
it can't use an index.

Julien, my concern was why the option 3 (with parallel) is not the fastest.
And now, even with parallel INSERT INTO .. SELECT its not the fastest. I
can't really use the UNLOGGED table in this case.

The following document summarises why is CREATE TABLE AS .. the fastest:
14.4.7
http://www.postgresql.org/docs/current/static/populate.html#POPULATE-PITR

Basically CREATE TABLE AS .. just doesn't write to wal if the wal_level is
minimal and hence cuts IO about in half.

On Tue, Jul 14, 2015 at 1:42 PM David Rowley <david.rowley@2ndquadrant.com>
wrote:

Marc, I am using postgres 9.4. I didn't benchmark, but intuitively the modulo operator will force traversing every record in table "a" 4 times, as it can't use an index.

Not necessarily. seq scans can be synchronized:

"This allows sequential scans of large tables to synchronize with each other, so that concurrent scans read the same block at about the same time and hence share the I/O workload. When this is enabled, a scan might start in the middle of the table and then "wrap around" the end to cover all rows, so as to synchronize with the activity of scans already in progress. This can result in unpredictable changes in the row ordering returned by queries that have no ORDER BY clause."

Marc

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