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

Hi Tom,

Here is what is actually there:

select count(*) from symbol_data where symbol_name='ELTE';
count
-------
687

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)

Interesting eh? EBALX also does a full scan--all others in the above
list get an index scan.

Here's the variable stuff--I attached the verbose outputs.
set enable_seqscan = on;
SET VARIABLE
vpm=> explain select * from symbol_data where symbol_name='ELTE';
NOTICE: QUERY PLAN:

Seq Scan on symbol_data (cost=0.00..707611.68 rows=355958 width=129)

set enable_seqscan = off;
SET VARIABLE
vpm=> explain select * from symbol_data where symbol_name='ELTE';
NOTICE: QUERY PLAN:

Index Scan using symbol_data_pkey on symbol_data (cost=0.00..1420899.51
rows=355958 width=129)

Index scan appears slower in explain, but the rows value is weird.

Thanks,
Michael

Tom Lane wrote:

>"Michael G. Martin" <michael(at)vpmonline(dot)com> writes:
>
>>Here is what I would expect which usually happens:
>>
>
>>explain select * from symbol_data where symbol_name='IBM';
>>Index Scan using symbol_data_pkey on symbol_data (cost=0.00..512.99 rows=128 width=129)
>>
>
>>Here is one that fails:
>>
>
>>explain select * from symbol_data where symbol_name='ELTE';
>>Seq Scan on symbol_data (cost=0.00..707415.32 rows=438015 width=129)
>>
>
>The planner thinks that there are 438K occurrences of 'ELTE' in your
>table. If that's true, a sequential scan is not obviously the wrong
>choice. How many are there, in reality? What do you get from
>
>select * from pg_stats where tablename = 'symbol_data' and attname =
>'symbol_name';
>
>Also, to put the rubber to the road: if you force an indexscan by
>doing "set enable_seqscan = off", does it get faster or slower?
>(EXPLAIN ANALYZE would be useful here.)
>
> regards, tom lane
>