From: Christian Weyer <christian.weyer@thinktecture.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Unexpected (bad) performance when querying indexed JSONB column
Date: Mon, 2 Feb 2015 06:31:24 +0000
Message-ID: <A191183A-AE61-40B7-84C5-355B7A429086@thinktecture.com> (raw)
In-Reply-To: <4791.1422829257@sss.pgh.pa.us>
References: <7F9BEAD7-C190-4E21-8240-04687FA88435@thinktecture.com>
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<54CE954F.7070904@agliodbs.com>
<BC4E7C7F-86CB-441A-ABCE-6E0837AA1B65@thinktecture.com>
<4791.1422829257@sss.pgh.pa.us>
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>>"Bitmap Heap Scan on articles (cost=16.25..135.64 rows=33 width=427)
>> (actual time=6.425..43.603 rows=18584 loops=1)"
>> " Recheck Cond: (data @> ‘{"locked": true}'::jsonb)"
>> " Heap Blocks: exact=1496"
>> " Buffers: shared hit=1504"
>> " -> Bitmap Index Scan on idx_data (cost=0.00..16.24 rows=33
>>width=0)
>> (actual time=6.090..6.090 rows=18584 loops=1)"
>> " Index Cond: (data @> ‘{"locked": true}'::jsonb)"
>> " Buffers: shared hit=8"
>> "Planning time: 0.348 ms"
>> "Execution time: 47.788 ms"
>
>So that's showing a runtime of 48 ms, not 900. For retrieving 18584
>rows, doesn't sound that bad to me.
>
>(If the planner had had a better rowcount estimate, it'd likely have
>not bothered with the index at all but just done a seqscan. This is
>a consequence of the lack of any very useful stats for JSONB columns,
>which is something we hope to address soon; but it's not done in 9.4
>and likely won't be in 9.5 either ...)
>
> regards, tom lane
Thanks for your insights.
This greatly helped.
-C.
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To: pgsql-performance@postgresql.org
Cc: christian.weyer@thinktecture.com, tgl@sss.pgh.pa.us, josh@agliodbs.com
Subject: Re: Unexpected (bad) performance when querying indexed JSONB column
In-Reply-To: <A191183A-AE61-40B7-84C5-355B7A429086@thinktecture.com>
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