: Cost calculation for EXPLAIN output
| От | Venkat Balaji |
|---|---|
| Тема | : Cost calculation for EXPLAIN output |
| Дата | |
| Msg-id | CAFrxt0iZrnNUkCjMQOm4JaOz8Y7wG4cd_26nHr2Op5cm9RxTag@mail.gmail.com обсуждение исходный текст |
| Ответы |
Re: : Cost calculation for EXPLAIN output
Re: : Cost calculation for EXPLAIN output |
| Список | pgsql-performance |
Hello,
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
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