Indexes and statistics - Mailing list pgsql-sql

Hi all,

This is a further post from last week. I've got a table of phone call detail records.

buns=# select count(*) from cdr;                          count
---------2800653
(1 row)

One of the customers is quite large (8.3% of the records):

buns=# select count(*) from cdr where cust_id = 99201110;        count
--------231889
(1 row)

I have indexes on cust_id (integer) and bill_id (integer). If I try to do a query on that customer it doesn't use the
index:

buns=# explain analyse select count(*) from cdr where cust_id = 99201110 and bill_id is null;
                      QUERY PLAN                                                     

-----------------------------------------------------------------------------------------------------------------Aggregate
(cost=87082.81..87082.81 rows=1 width=0) (actual time=82279.63..82279.63 rows=1 loops=1)  ->  Seq Scan on cdr
(cost=0.00..87037.71rows=18041 width=0) (actual time=82279.61..82279.61 rows=0 loops=1)        Filter: ((cust_id =
99201110)AND (bill_id IS NULL))Total runtime: 82280.19 msec 
(4 rows)

I tried this:

alter table cdr alter column cust_id set statistics 1000;
alter table cdr alter column bill_id set statistics 1000;
analyze verbose;

The I ran the query again but I still got the same result.

Then I tried disabling sequential scans in postgresql.conf, restarted the postmaster and did the query again:

buns=# explain analyse select count(*) from cdr where cust_id = 99201110 and bill_id is null;
                               QUERY PLAN                                                             

----------------------------------------------------------------------------------------------------------------------------------Aggregate
(cost=913498.60..913498.60 rows=1 width=0) (actual time=48387.91..48387.91 rows=1 loops=1)  ->  Index Scan using
cdr_ix3on cdr  (cost=0.00..913453.49 rows=18041 width=0) (actual time=48387.89..48387.89 rows=0 loops=1)        Index
Cond:(cust_id = 99201110)        Filter: (bill_id IS NULL)Total runtime: 48388.47 msec 
(5 rows)

The computed cost of using the index was a factor of 10 higher which I presume is why the query planner wasn't using
theindex, but it ran in half the time. So I guess I need to know how to alter the statistics collection so that the
indexwill get used. I gather that index columns that occur in more than "a few" percent of the table cause the query
plannerto not use the index. Does that mean I won't be able to get the query planner to ever use the cust_id index for
thatcustomer or can I tune some parameters to alter that? 

Any suggestions appreciated.

Thanks,
David

David Witham
Telephony Platforms Architect
Unidial, Australia