Re: query performance with hstore vs. non-hstore

Hi David,

Thanks for the reply.

>Calling 43s "close to" 70s doesn't sound right...

Oops, I'm not saying 43s close to 70s... I mean that the plan generated by disable timing for explain plan doesn't make obvious difference comparing to the earlier plan I sent out which enabled timing.

>What version of PostgreSQL are you using?
>
>Two calls to each() and cast to numeric are not free.
>
>Your sequential scan savings is nearly 9 seconds but you lose all of that, and more, when PostgreSQL evaluates the result of the scan and has to process the each() and >the cast before it performs the join against the expanded result. There is no planner node for this activity but it does cost time - in this case more time than it >would take to simply store the native data types in separate rows.
>
>You really should expand the hstore after the join (i.e., in the top-most
>select-list) but in this case since the join removed hardly any rows the gain from doing so would be minimal. The idea being you should not expand the hstore of any row >that fails the join condition since it will not end up in the final result anyway.
>
>Also, in this specific case, the call to each(...).key is pointless - you never use the data.
>
>If you did need to use both columns, and are using 9.3, you should re-write this to use LATERAL.
>
>In 9.2- you, possibly using a CTE, could do something like this:
>
>SELECT (each).* FROM (
>SELECT each(hs) FROM ( VALUES('k=>1'::hstore) ) h (hs)
>) src
>
>This is a single call to each(), in a subquery, which result is then expanded using (col).* notation in the parent query. This avoids calling each twice - and note that >(each(...).*) does not work to avoid the double-call - you have to use a subquery / a CTE one to ensure that it is not collapsed (offset 0 should work too but I find the >CTE one a little cleaner personally).
>

I'm using Postgresql 9.3.4.
I changed the query as you suggested. The execution time are still similar to the original one.

dev=# explain analyze select (each).key as cha_type, sum((each).value::numeric) as visits from (select each(visits) from weekly_hstore a join seg1 b on a.ref_id=b.ref_id )foo group by cha_type order by visits desc;
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------------
Sort (cost=9455046.69..9455047.19 rows=200 width=32) (actual time=70928.881..71425.833 rows=3639539 loops=1)
Sort Key: (sum(((foo.each).value)::numeric))
Sort Method: quicksort Memory: 394779kB
-> HashAggregate (cost=9455037.05..9455039.05 rows=200 width=32) (actual time=60077.937..61425.469 rows=3639539 loops=1)
-> Subquery Scan on foo (cost=12029.58..5737447.05 rows=371759000 width=32) (actual time=281.658..23912.400 rows=36962761 loops=1)
-> Hash Join (cost=12029.58..2019857.05 rows=371759000 width=186) (actual time=281.655..18759.265 rows=36962761 loops=1)
Hash Cond: ((a.ref_id)::text = (b.ref_id)::text)
-> Seq Scan on weekly_hstore a (cost=0.00..133321.14 rows=1292314 width=232) (actual time=11.141..857.959 rows=1292314 loops=1)
-> Hash (cost=7382.59..7382.59 rows=371759 width=47) (actual time=262.722..262.722 rows=371759 loops=1)
Buckets: 65536 Batches: 1 Memory Usage: 28951kB
-> Seq Scan on seg1 b (cost=0.00..7382.59 rows=371759 width=47) (actual time=11.701..113.859 rows=371759 loops=1)
Total runtime: 71626.871 ms
(12 rows)

-----Original Message-----
From: pgsql-performance-owner(at)postgresql(dot)org [mailto:pgsql-performance-owner(at)postgresql(dot)org] On Behalf Of David G Johnston
Sent: Tuesday, September 02, 2014 1:38 PM
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] query performance with hstore vs. non-hstore

Huang, Suya wrote
> See output of explain (analyze,timing off), the total runtime is close
> to the one enable timing.

Calling 43s "close to" 70s doesn't sound right...

> dev=# explain (analyze, timing off) select cha_type, sum(visits) from
> (select (each(visits)).key as cha_type,(each(visits)).value::numeric
> as visits from weekly_hstore a join seg1 b on a.ref_id=b.ref_id )foo
> group by cha_type order by sum(visits) desc;

What version of PostgreSQL are you using?

Two calls to each() and cast to numeric are not free.

Your sequential scan savings is nearly 9 seconds but you lose all of that, and more, when PostgreSQL evaluates the result of the scan and has to process the each() and the cast before it performs the join against the expanded result. There is no planner node for this activity but it does cost time - in this case more time than it would take to simply store the native data types in separate rows.

You really should expand the hstore after the join (i.e., in the top-most
select-list) but in this case since the join removed hardly any rows the gain from doing so would be minimal. The idea being you should not expand the hstore of any row that fails the join condition since it will not end up in the final result anyway.

Also, in this specific case, the call to each(...).key is pointless - you never use the data.

If you did need to use both columns, and are using 9.3, you should re-write this to use LATERAL.

In 9.2- you, possibly using a CTE, could do something like this:

SELECT (each).* FROM (
SELECT each(hs) FROM ( VALUES('k=>1'::hstore) ) h (hs)
) src

This is a single call to each(), in a subquery, which result is then expanded using (col).* notation in the parent query. This avoids calling each twice - and note that (each(...).*) does not work to avoid the double-call - you have to use a subquery / a CTE one to ensure that it is not collapsed (offset 0 should work too but I find the CTE one a little cleaner personally).

David J.

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