Re: Measuring the Query Optimizer Effect: Turning off the QO?

Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
>  Tom> Two I'd particularly draw your attention to are
>  Tom> join_collapse_limit and from_collapse_limit --- if you set both to
>  Tom> 1, that'll effectively disable searching for a good join order,
>  Tom> causing the join order to match the syntactic structure of the
>  Tom> FROM clause. For instance "FROM a,b,c" will always be done by
>  Tom> joining a to b first

> FROM a,b,c can always be planned in any join order.

Ah, right, -ENOCAFFEINE.  What from_collapse_limit really does is
prevent flattening sub-SELECTs when doing so would add more join-order
freedom in the parent query.  But ignoring sub-SELECTs, using explicit
JOIN syntax with join_collapse_limit=1 will let you control the
join order.

            regards, tom lane