Re: Slow index scan - Pgsql 9.2

2017-01-11 4:05 GMT+13:00 Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>:

> On 01/10/2017 04:05 AM, Patrick B wrote:
>
>> ​3,581​ individual pokes into the heap to confirm tuple visibility
>> and apply the deleted filter - that could indeed take a while.
>> David J.
>>
>>
>> I see.. The deleted column is:
>>
>> deleted boolean
>>
>> Should I create an index for that? How could I improve this query?
>>
>>
>> Does it execute as slowly when you run it for a 2nd time?
>>
>>
>> No, it doesn't. I think it's because of cache?
>>
>>
>> I would think because of the NOT "deleted" clause. Which is
>> interesting, because that's a column which you conveniently didn't
>> include in the definition below.
>>
>>
>> My mistake.
>>
>>
>> Would an Index be sufficient to solve the problem?
>>
>>
> Not a separate index - the query probably would not benefit from two
> separate indexes. But you can amend the existing index, to allow index-only
> scans, i.e. creating an index like this:
>
> CREATE INDEX ON (clientid, is_demo, deleted, id, job, job_share_mode)
>
> This will make the index larger, but it should allow index-only scans.
>
> The other thing you could try is partial index, i.e.
>
> CREATE INDEX ON (clientid) WHERE NOT is_demo AND NOT deleted;
>
> You can also combine those approaches, but you'll have to include all
> columns into the index, even those in the index predicate:
>
> CREATE INDEX ON (clientid, is_demo, deleted, id, job, job_share_mode)
> WHERE NOT is_demo AND NOT deleted;
>
> I'd bet all of those will outperform the current plan.
>
> regards
>
> --
> Tomas Vondra http://www.2ndQuadrant.com
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
>

Thanks for the reply!

I decided to create a partial index for that query, as it is part of a much
bigger one and it is run at all the time.

Patrick