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

On 08/07/2019 18:29, Tom Lane wrote:
> Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
>> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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
>

Thanks for the clarification. I really appreciate taking the time to
provide such in depth info about my query!