I have a strange performance issue, i think that is not possible:
Given this statement:
SELECT several_fields FROM A, B, C WHERE conditions
A, B are tables with several LEFT JOINS but they act as one subquery.
If I execute the select above:
SELECT several_fields FROM A, B, C WHERE conditions
Time: 30 secs
Cost: 1M
If I execute the same select (same parameters) but swapping A and B in the from clause:
SELECT several_fields FROM B, A, C WHERE conditions
Cost: 10k
The plan changes dramatically: I can't see why the order of FROM clause impacts directly on the query cost and plan. If this is possible, where i can read about it? I need to know how the order of FROM clause modifies the query plan.
Thanks in advance. This is my first post.
Eduard Català