Re: Why does this query write to the disk?

How big is your work_mem setting, and is this behavior affected by its size?

You can increase the work_mem on an individual connection before the test.

Simply:

set work_mem = '100MB'

to set it to 100 Megabytes. If your issue is spilling data out of work_mem
to the temp storage, this setting will affect that.

On Thu, Sep 18, 2008 at 10:30 AM, Nikolas Everett <nik9000(at)gmail(dot)com> wrote:

> List,
>
> I'm a bit confused as to why this query writes to the disk:
> SELECT count(*)
> FROM bigbigtable
> WHERE customerid IN (SELECT customerid FROM
> smallcustomertable)
> AND x !=
> 'special'
>
> AND y IS NULL
>
> It writes a whole bunch of data to the disk that has the tablespace where
> bigbigtable lives as well as writes a little data to the main disk. It
> looks like its is actually WAL logging these writes.
>
> Here is the EXPLAIN ANALYZE:
> Aggregate (cost=46520194.16..46520194.17 rows=1 width=0) (actual
> time=4892191.995..4892191.995 rows=1 loops=1)
> -> Hash IN Join (cost=58.56..46203644.01 rows=126620058 width=0)
> (actual time=2.938..4840349.573 rows=79815986 loops=1)
> Hash Cond: ((bigbigtable.customerid)::text =
> (smallcustomertable.customerid)::text)
> -> Seq Scan on bigbigtable (cost=0.00..43987129.60 rows=126688839
> width=11) (actual time=0.011..4681248.143 rows=128087340 loops=1)
> Filter: ((y IS NULL) AND ((x)::text <> 'special'::text))
> -> Hash (cost=35.47..35.47 rows=1847 width=18) (actual
> time=2.912..2.912 rows=1847 loops=1)
> -> Seq Scan on smallcustomertable (cost=0.00..35.47
> rows=1847 width=18) (actual time=0.006..1.301 rows=1847 loops=1)
> Total runtime: 4892192.086 ms
>
> Can someone point me to some documentation as to why this writes to disk?
>
> Thanks,
> Nik
>