Обсуждение: Re: [COMMITTERS] pgsql: If we expect a hash join to be performed in multiple batches,

On Thu, 2009-03-26 at 17:15 +0000, Tom Lane wrote:
> Log Message:
> -----------
> If we expect a hash join to be performed in multiple batches, suppress
> "physical tlist" optimization on the outer relation (ie, force a projection
> step to occur in its scan).  This avoids storing useless column values when
> the outer relation's tuples are written to temporary batch files.

Can we add "batches=N" to the EXPLAIN output for Hash and/or Hash Join?

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support

Simon Riggs <simon@2ndquadrant.com> writes:
> Can we add "batches=N" to the EXPLAIN output for Hash and/or Hash Join?

Are you talking about expected batches, or actual batches?  If the
former, would it be sufficient to distinguish "1" from "more than 1"?
If so, maybe changing the node title to "Batched Hash" would do.
        regards, tom lane

On Thu, 2009-04-02 at 15:49 -0400, Tom Lane wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
> > Can we add "batches=N" to the EXPLAIN output for Hash and/or Hash Join?
> 
> Are you talking about expected batches, or actual batches?  

Expected batches for EXPLAIN, both for EXPLAIN ANALYZE.

> If the
> former, would it be sufficient to distinguish "1" from "more than 1"?
> If so, maybe changing the node title to "Batched Hash" would do.

Hmmm, knowing the number of batches is beneficial since it helps you to
calculate the required memory to get best performance.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support