Re: Indexes not always used after inserts/updates/vacuum analyze

yes.  each symbol_name only gets one row added and maybe a few updated
each market day.
This is interesting too.  Planner thinks 128 rows on this symbol, GE,
yet there are really 5595.  Not as off as ELTE, but a large factor.  at
least the index get hit here.

explain select * from symbol_data where symbol_name='GE';
NOTICE:  QUERY PLAN:

Index Scan using symbol_data_pkey on symbol_data  (cost=0.00..513.09
rows=128 width=129)

EXPLAIN
vpm=> select count(*) from  symbol_data where symbol_name='GE';
 count
-------
  5595



Tom Lane wrote:

>"Michael G. Martin" <michael@vpmonline.com> writes:
>
>>Here is what is actually there:
>>
>
>> select count(*) from symbol_data where symbol_name='ELTE';
>>   687
>>
>
>Hmm.  Do you have reason to think that that was also true when you last
>did VACUUM ANALYZE or VACUUM?
>
>>Here is the pg_stat query:
>> select * from pg_stats where tablename = 'symbol_data' and attname
>>='symbol_name';
>>  tablename  |   attname   | null_frac | avg_width | n_distinct
>>|               most_common_vals
>>|
>>most_common_freqs
>>|                  histogram_bounds                   | correlation

>>-------------+-------------+-----------+-----------+------------+----------------------------------------------+---------------------------------------------------------------------------------------------------------------+-----------------------------------------------------+-------------
>> symbol_data | symbol_name |         0 |         7 |     152988 |
>>{EBALX,ELTE,LIT,OEX,RESC,BS,ESH,HOC,IBC,IDA} |
>>{0.0183333,0.0173333,0.00166667,0.00166667,0.00166667,0.00133333,0.00133333,0.00133333,0.00133333,0.00133333}
>>| {A,BMO,DBD,FSCHX,IIX,MAS,NSANY,PTEC,SR,UTIL,_^^VPM} |    0.128921
>>(1 row)
>>
>
>What this says is that in the last ANALYZE, EBALX accounted for 18% of
>the sample, and ELTE for 17%.  Does that seem plausible to you?  If the
>sample was accurate then I'd agree with the planner's choices.  It'd
>seem that either your table contents are changing drastically (in which
>case more-frequent ANALYZEs may be the answer), or you had the bad luck
>to get a very unrepresentative sample, or there's some bug in the
>statistical calculations.
>
>            regards, tom lane
>