Re: query a table with lots of coulmns - Mailing list pgsql-performance

>At first, thanks for your fast and comprehensive help.
>
>The structure of my cache table is
>
>a text , b text NOT NULL , c text , d text , e timestamp without
>timezone DEFAULT now(),  f text, s1 integer DEFAULT 0, s2 integer
>DEFAULT 0, s3 integer DEFAULT 0, ... ,s512 DEFAULT 0


>additional constraints: primary key (b)  , Unique(b), Unique(a)
>Indexes : Index on a, Index on b

This looks redundant. e.g. you don't need a unique index on b if you already have a primary key on it.
Can you post the complete table definition ?

...

>One remark which might help: overall 90 - 95 % of the s1-s512 columns
>are 0. I am only interested in columns not equals 0. Perhaps it would
>make sense to use and array of json and enumerate only values not equals 0.

Could you change that to replace 0 values with NULLs?
This would greatly reduce your table space as Postgres is very efficient about NULLs storage:
It marks all null values in a bit map within the row header so you just need about one bit per null
instead of 4 bytes for zeros, and hence get rid of your I/O issue.

regards,

Marc Mamin
________________________________________
Von: pgsql-performance-owner@postgresql.org [pgsql-performance-owner@postgresql.org]" im Auftrag von "Björn
Wittich[Bjoern_Wittich@gmx.de] 
Gesendet: Samstag, 20. September 2014 09:19
An: Josh Berkus; pgsql-performance@postgresql.org
Betreff: Re: [PERFORM] query a table with lots of coulmns

At first, thanks for your fast and comprehensive help.

The structure of my cache table is

a text , b text NOT NULL , c text , d text , e timestamp without
timezone DEFAULT now(),  f text, s1 integer DEFAULT 0, s2 integer
DEFAULT 0, s3 integer DEFAULT 0, ... ,s512 DEFAULT 0

additional constraints: primary key (b)  , Unique(b), Unique(a)
Indexes : Index on a, Index on b

This table has 30 Mio rows ( will increase to 50 Mio) in future

My working table is

b text, g integer

Indexes on b and c


This table has 5 Mio rows

Scenario:

What I want to achieve :

SELECT s1,s2,s3,...s512,g,d  from <worktable> INNER JOIN <cachetable>
USING(b) ORDER BY g


The inner join will match at least 95 % of columns of the smaller
worktable in this example 4,75 mio rows.

Running this query takes several hours until I receive the first
results. Query analyzing shows that the execution plan is doing 2 seq
table scans on cache and work table.


When I divide this huge statement into

SELECT s1,s2,s3,...s512,g,d  from <worktable> INNER JOIN <cachetable>
USING(b) WHERE g BETWEEN 1 and 10000 ORDER BY g, SELECT
s1,s2,s3,...s512,g,d  from <worktable> INNER JOIN <cachetable> USING(b)
WHERE g BETWEEN 10001 and 20000 ORDER BY g, ....

(I can do this because g i unique and continous id from 1 to N)

The result is fast but fireing parallel requests (4-8 times parallel)
slows down the retrieval.

Execution plan changes when adding "BETWEEN 1 and 10000" to use the indexes.



One remark which might help: overall 90 - 95 % of the s1-s512 columns
are 0. I am only interested in columns not equals 0. Perhaps it would
make sense to use and array of json and enumerate only values not equals 0.

Statistics on the large table:
table size: 80 GB
toast-tablesize: 37 GB
size of indexes: 17 GB


Thanks for your help and ideas

Björn





Am 19.09.2014 23:40, schrieb Josh Berkus:
> On 09/19/2014 04:51 AM, Björn Wittich wrote:
>> I am relatively new to postgres. I have a table with 500 coulmns and
>> about 40 mio rows. I call this cache table where one column is a unique
>> key (indexed) and the 499 columns (type integer) are some values
>> belonging to this key.
>>
>> Now I have a second (temporary) table (only 2 columns one is the key of
>> my cache table) and I want  do an inner join between my temporary table
>> and the large cache table and export all matching rows. I found out,
>> that the performance increases when I limit the join to lots of small
>> parts.
>> But it seems that the databases needs a lot of disk io to gather all 499
>> data columns.
>> Is there a possibilty to tell the databases that all these colums are
>> always treated as tuples and I always want to get the whole row? Perhaps
>> the disk oraganization could then be optimized?
> PostgreSQL is already a row store, which means by default you're getting
> all of the columns, and the columns are stored physically adjacent to
> each other.
>
> If requesting only 1 or two columns is faster than requesting all of
> them, that's pretty much certainly due to transmission time, not disk
> IO.  Otherwise, please post your schema (well, a truncated version) and
> your queries.
>
> BTW, in cases like yours I've used a INT array instead of 500 columns to
> good effect; it works slightly better with PostgreSQL's compression.
>



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