Table order at FROM clause affects performance?

[Eduard Català <eduard.catala@xxxxxxxxx>]

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

Time: 19ms

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