On 12 December 2012 15:33, Evgeny Shishkin
<itparanoia@xxxxxxxxx> wrote:
Optimiser thinks that nested loop is more expensive, because of point PK lookups, which a random io.
Can you set random_page_cost to 2 or 3 and try again?
Hi Evgeny
Thanks for the quick reply. Setting random_page_cost to 3 doesn't make a difference, but to 2 makes the optimiser to choose nested loop. However, with such a small penalty for random I/O, I'm worry about this setting will make other small queries incorrectly use index when it should be a sequential scan though. I understand random I/O is expensive, but in this case the optimiser already knows the big table is really big, should it consider a sequential scan will be slower than an index lookup? Scan 170 million records vs index lookup of 50,000 records. Any thoughts?
Yes, this is the most common issue for me.
Usually you just have to find the right combination of random and seq scan costs, shared_buffers and effective_cache_size.
If some of the queries work well with another value of, say, random_page_cost, then, since it is per session parameter, you can SET it in your session before the query. But over time your table may change in size and distribution and everything brakes. No speaking about general ugliness from application standpoint.
May be somebody more experienced would help.
Also you can set different costs per tablespace.
Thanks
Huan
