Message-ID: <548614C8.90203@aklaver.com>
Date: Mon, 08 Dec 2014 13:14:48 -0800
From: Adrian Klaver <adrian.klaver@aklaver.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: Tim Dudgeon <tdudgeon.ml@gmail.com>, pgsql-sql@postgresql.org,
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Subject: Re: querying with index on jsonb slower than standard column.
 Why?
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On 12/08/2014 12:53 PM, Tom Lane wrote:
> I wrote:
>> Adrian Klaver <adrian.klaver@aklaver.com> writes:
>>> Seems work_mem is the key:
> 
>> Fascinating.  So there's some bad behavior in the lossy-bitmap stuff
>> that's exposed by one case but not the other.
> 
> Meh.  I was overthinking it.  A bit of investigation with oprofile exposed
> the true cause of the problem: whenever the bitmap goes lossy, we have to
> execute the "recheck" condition for each tuple in the page(s) that the
> bitmap has a lossy reference to.  So in the fast case we are talking about
> 
> Recheck Cond: ((assay1_ic50 > 90::double precision) AND (assay2_ic50 < 10::double precision))
> 
> which involves little except pulling the float8 values out of the tuple
> and executing float8gt and float8lt.  In the slow case we have got
> 
> Recheck Cond: ((((data ->> 'assay1_ic50'::text))::double precision > 90::double precision) AND (((data ->> 'assay2_ic50'::text))::double precision < 10::double precision))
> 
> which means we have to pull the JSONB value out of the tuple, search
> it to find the 'assay1_ic50' key, convert the associated value to text
> (which is not exactly cheap because *the value is stored as a numeric*),
> then reparse that text string into a float8, after which we can use
> float8gt.  And then probably do an equivalent amount of work on the way
> to making the other comparison.
> 
> So this says nothing much about the lossy-bitmap code, and a lot about
> how the JSONB code isn't very well optimized yet.  In particular, the
> decision not to provide an operator that could extract a numeric field
> without conversion to text is looking pretty bad here.
> 

I think I understand the above.

I redid the test on my 32-bit machine, setting work_mem=16MB, and I got comparable results
to what I saw on the 64-bit machine. So, what I am still am puzzled by is why work_mem seems 
to make the two paths equivalent in time?:

Fast case, assay1_ic50 > 90 and assay2_ic50 < 10:
1183.997 ms

Slow case, (data->>'assay1_ic50')::float > 90 and (data->>'assay2_ic50')::float < 10;:
1190.187 ms

> 
> 			regards, tom lane
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com


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