You're right : my question is why the planner doesn't use the index ! My DELETE statements have WHERE clause like : start_date<1346486100000. They are executed to delete too old rows.
My postgresql version is 8.4. Below is an example of a table (they all have the same structure) :
CREATE TABLE agg_t100_outgoing_a39_src_net_f5
(
total_pkts bigint,
end_date bigint,
src_network inet,
start_date bigint,
total_flows bigint,
total_bytes bigint
)
WITH (
OIDS=FALSE
);
CREATE INDEX agg_t100_outgoing_a39_src_net_f5_end_date
ON agg_t100_outgoing_a39_src_net_f5
USING btree
(end_date);
CREATE INDEX agg_t100_outgoing_a39_src_net_f5_start_date
ON agg_t100_outgoing_a39_src_net_f5
USING btree
(start_date);
I have investigated in the pg_stat_all_tables table and it seems the autovaccum / autoanalyze don't do their job. Many tables have no last_autovacuum / last_autoanalyze dates ! So the planner doesn't have fresh stats to decide. Don't you think it could be a good reason for slow DELETE ? In this case, the trouble could come from the autovaccum configuration.
Regards,
Sylvain
Hi Sylvain,
Might sound like a nasty question, and gurus will correct me if I'm wrong, but first thing to investigate is why the index is not used :
- You have 2/3 million rows per table so the planner should use the index. Seqscan is prefered for small tables.
- Maybe the WHERE clause of your DELETE statement doesn't make use of your start and end date columns ? If so, in which order ?
Please, provide with your Pg version and the table setup with the index.
I've got a trouble with some delete statements. My db contains a little more than 10000 tables and runs on a dedicated server (Debian 6 - bi quad - 16Gb - SAS disks raid 0). Most of the tables contains between 2 and 3 million rows and no foreign keys exist between them. Each is indexed (btree) on start_date / end_date fields (bigint). The Postgresql server has been tuned (I can give modified values if needed).
I perform recurrent DELETE upon a table subset (~1900 tables) and each time, I delete a few lines (between 0 and 1200). Usually it takes between 10s and more than 2mn. It seems to me to be a huge amount of time ! An EXPLAIN ANALYZE on a DELETE shows me that the planner uses a Seq Scan instead of an Index Scan. Autovaccum is on and I expect the db stats to be updated in real time (pg_stats file is stored in /dev/shm RAM disk for quick access).
