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Re: Update with Subquery Performance

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Linux Guru" <linux(dot)binary(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Update with Subquery Performance
Date: 2008-02-12 16:18:05
Message-ID: 27294.1202833085@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
"Linux Guru" <linux(dot)binary(at)gmail(dot)com> writes:
> Analyzing did not help, here is the out of EXPLAIN ANALYZE of update query
> "Seq Scan on dummy  (cost=0.00..56739774.24 rows=23441 width=275) (actual
> time=18.927..577929.014 rows=22712 loops=1)"
> "  SubPlan"
> "    ->  Aggregate  (cost=2420.41..2420.43 rows=1 width=19) (actual time=
> 25.423..25.425 rows=1 loops=22712)"
> "          ->  Seq Scan on dummy "temp"  (cost=0.00..2416.01 rows=586
> width=19) (actual time=0.049..17.834 rows=2414 loops=22712)"
> "                Filter: ((product)::text = ($0)::text)"
> "Total runtime: 578968.885 ms"

Yeah, that's just not going to be fast.  An index on the product column
might help a bit, but the real issue is that you're repetitively
calculating the same aggregates.  I think you need a separate temp
table, along the lines of

create temp table dummy_agg as
  select product,
         (case when sum(pd) <> 0 then sum(gd)/sum(pd)*100 else 0 end) as s
  from dummy
  group by product;

create index dummy_agg_i on dummy_agg(product); -- optional

update dummy
  set gp= (select s from dummy_agg where dummy_agg.product = dummy.product);

The index would only be needed if you expect a lot of rows (lot of
different product values).

			regards, tom lane

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