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@vpmonline.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