Re: ATTACH/DETACH PARTITION CONCURRENTLY

Thanks for this work!  I like the name "partition directory".

On 2018-Dec-20, Robert Haas wrote:

> 0002 introduces the concept of a partition directory.  The idea is
> that the planner will create a partition directory, and so will the
> executor, and all calls which occur in those places to
> RelationGetPartitionDesc() will instead call
> PartitionDirectoryLookup().  Every lookup for the same relation in the
> same partition directory is guaranteed to produce the same answer.  I
> believe this patch still has a number of weaknesses.  More on that
> below.

The commit message for this one also points out another potential
problem,

> Introduce the concept of a partition directory.
>
> Teach the optimizer and executor to use it, so that a single planning
> cycle or query execution gets the same PartitionDesc for the same table
> every time it looks it up.  This does not prevent changes between
> planning and execution, nor does it guarantee that all tables are
> expanded according to the same snapshot.

Namely: how does this handle the case of partition pruning structure
being passed from planner to executor, if an attach happens in the
middle of it and puts a partition in between existing partitions?  Array
indexes of any partitions that appear later in the partition descriptor
will change.

This is the reason I used the query snapshot rather than EState.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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