I am trying to understand the analysis behind the "cost" attribute in EXPLAIN output.
postgres = # explain select * from table_event where seq_id=8520960;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
Index Scan using te_pk on table_event (cost=0.00..13.88 rows=1 width=62)
Index Cond: (sequence_id = 8520960)
The cost is "13.88" to fetch 1 row by scanning an Primary Key indexed column.
Isn't the cost for fetching 1 row is too high ?
On the same table, the cost calculation for scanning the full table is looking justified --
postgres=# explain select * from table_event;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on table_event (cost=0.00..853043.44 rows=38679544 width=62)
(1 row)
(disk pages read * seq_page_cost) + (rows scanned * cpu_tuple_cost) = (466248 * 1) + (38679544 * 0.01) = 853043.44
By the way below are the details -
Version - Postgres-9.0
Table size is - 3643 MB
+Indexes the size is - 8898 MB
I am looking for a way to reduce cost as much as possible because the query executes 100000+ times a day.
Any thoughts ?
Thanks,
VB
