Re: [PERFORM] Slow query question

[Andrey Povazhnyi]

Tom,

Thank you for a thorough answer. We’ll try the 2-column index.

Regards,
Andrey Povazhnyi

> On Dec 6, 2016, at 6:33 PM, Tom Lane  wrote:
> 
> Andrey Povazhnyi  writes:
>> We’ve got a strange planner behavior on a query to one of our bigger tables 
>> after we upgraded to postgres 9.6.1 recently.
> 
> The basic problem with this query is that there are no good alternatives.
> The planner believes there are about 53K rows matching the WHERE
> condition.  (I assume this estimate is roughly in line with reality,
> else we have different problems to talk about.)  It can either scan down
> the "id" index and stop when it finds the 30th row matching WHERE, or
> it can use the "symbol" index to read all 53K rows matching WHERE and
> then sort them by "id".  Neither one of those is going to be speedy;
> but the more rows there are matching WHERE, the better the first way
> is going to look.
> 
> If you're worried about doing this a lot, it might be worth your while
> to provide a 2-column index on (source, id) --- in that order --- which
> would allow a query plan that directly finds the required 30 rows as
> consecutive index entries.  Possibly this could replace your index on
> "source" alone, depending on how much bigger the 2-col index is and
> how many queries have no use for the second column.  See
> https://www.postgresql.org/docs/current/static/indexes.html
> particularly 11.3 - 11.5.
> 
>   regards, tom lane



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Re: [PERFORM] Slow query question

[Tom Lane]

Andrey Povazhnyi  writes:
> We’ve got a strange planner behavior on a query to one of our bigger tables 
> after we upgraded to postgres 9.6.1 recently.

The basic problem with this query is that there are no good alternatives.
The planner believes there are about 53K rows matching the WHERE
condition.  (I assume this estimate is roughly in line with reality,
else we have different problems to talk about.)  It can either scan down
the "id" index and stop when it finds the 30th row matching WHERE, or
it can use the "symbol" index to read all 53K rows matching WHERE and
then sort them by "id".  Neither one of those is going to be speedy;
but the more rows there are matching WHERE, the better the first way
is going to look.

If you're worried about doing this a lot, it might be worth your while
to provide a 2-column index on (source, id) --- in that order --- which
would allow a query plan that directly finds the required 30 rows as
consecutive index entries.  Possibly this could replace your index on
"source" alone, depending on how much bigger the 2-col index is and
how many queries have no use for the second column.  See
https://www.postgresql.org/docs/current/static/indexes.html
particularly 11.3 - 11.5.

regards, tom lane


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