Re: Partition-wise join for join between (declaratively) partitioned tables

On Wed, Oct 4, 2017 at 11:34 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> + Enables or disables the query planner's use of partition-wise join
> + plans. When enabled, it spends time in creating paths for joins between
> + partitions and consumes memory to construct expression nodes to be used
> + for those joins, even if partition-wise join does not result in the
> + cheapest path. The time and memory increase exponentially with the
> + number of partitioned tables being joined and they increase linearly
> + with the number of partitions. The default is <literal>off</>.
>
> I think this is too scary and too much technical detail. I think you
> could just say something like: Enables or disables use of
> partition-wise join, which allows a join between partitioned tables to
> be performed by joining the matching partitions. Partition-wise join
> currently applies only when the join conditions include all the
> columns of the partition keys, which must be of the same data type and
> have exactly matching sets of child partitions. Because
> partition-wise join planning can use significantly increase CPU time
> and memory usage during planning, the default is <literal>off</>.

Not enough caffeine, obviously: should have been something like --
Because partition-wise join can significantly increase the CPU and
memory costs of planning...

--
Robert Haas
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