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Make TOAST_TUPLES_PER_PAGE configurable per table.

From: Jesper Krogh <jesper(at)krogh(dot)cc>
To: PostgreSQL-development Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Make TOAST_TUPLES_PER_PAGE configurable per table.
Date: 2010-02-01 21:33:49
Message-ID: 4B6748BD.5050003@krogh.cc (view raw or flat)
Thread:
Lists: pgsql-hackers
Hi

This is my first attempt to hack PostgreSQL (even C actually), so bear
over with obvious mistakes done.

I've had a wish to be able to teach Postgres a bit more about how to
store its data on disk. Our systems is a typical web-based system where
all access more or less can be devided into 2 categories:
"List view" .. which is overview, counts, aggregates on simple values
with 50..200 rows per page and
"details views" which is more or less all data from a single rows
combined with aggregates of relations and similar.

Bases on this knowledge I know that there is a significant amount of
data stored "inline" in tuples and being read of disk for the listing
that is "never needed". At the moment it'll try to compress an get below
pagesize/4 ~ 2KB/tuple before it gets out to TOASTING the large tables.

Looking at the current implementation it seems to "do the right thing"
since the "large, variable length" attributes are the "most likely" to
not be shown on listing pages anyway, but it is not aggressive enough
(in my view for all common web-things), so this patch tries to make
TOAST_TUPLES_PER_PAGE per table configurable (the desired tuple-density
on the main storage).

This patch enables users to set TOAST_TUPLES_PER_PAGE with

ALTER TABLE <table> SET (tuples_per_page = X); .. currently with 1 <= X
<= 32;

ftstest=# create table testtext8(id SERIAL,col text) with
(tuples_per_page=8);
NOTICE:  CREATE TABLE will create implicit sequence "testtext8_id_seq"
for serial column "testtext8.id"
CREATE TABLE
ftstest=# create table testtext2(id SERIAL,col text) with
(tuples_per_page=2);
NOTICE:  CREATE TABLE will create implicit sequence "testtext2_id_seq"
for serial column "testtext2.id"
CREATE TABLE
ftstest=# insert into testtext8(col) (select (select
array_to_string(array_agg(chr((random()*95+30)::integer)),'') from
generate_series(1,3000)) as testtext from generate_series(1,50000));
INSERT 0 50000
ftstest=# insert into testtext2(col) (select (select
array_to_string(array_agg(chr((random()*95+30)::integer)),'') from
generate_series(1,3000)) as testtext from generate_series(1,50000));
INSERT 0 50000
ftstest=# \timing
### Here i stop PG and echo 3 > /proc/sys/vm/drop_caches
ftstest=# select count(id) from testtext2;
FATAL:  terminating connection due to administrator command
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
ftstest=# select count(id) from testtext2;
 count
-------
 50000
(1 row)

Time: 4613.044 ms
ftstest=# select count(id) from testtext8;
 count
-------
 50000
(1 row)

Time: 318.743 ms

This obviously comes with a drawback if I actually "Need" the data.

ftstest=# select max(length(col)) from testtext2;
 max
------
 3000
(1 row)

Time: 1445.016 ms
ftstest=# select max(length(col)) from testtext8;
 max
------
 3000
(1 row)

Time: 4184.994 ms

            relation             |    size
---------------------------------+------------
 pg_toast.pg_toast_1450869       | 195 MB
 public.testtext2                | 195 MB
 public.testtext8                | 2552 kB


No documentation on the patch. I'll do that a bit later.

Generally speaking.. if you have some knowledge about the access
patterns of your data then this patch can enable you to teach postgresql
to take advantage of that. In my situation I would estimate that the
production set would be able to drop a couple of GB from main memory
(leaving room for more index-pages and such).


Thanks in advance.

-- 
Jesper Krogh

Attachment: tuples-per-page.patch
Description: text/x-patch (3.1 KB)

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