Re: Temporary tables under hot standby

On Wed, Apr 25, 2012 at 12:18 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Apr 25, 2012 at 12:08 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
>> On Wed, Apr 25, 2012 at 4:49 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>>> How important is support for VACUUM on these tables under hot standby?  The
>>>> alternative is to fail when a session retains a temporary table across 2B
>>>> local transactions.  I do not currently see any challenges sufficient to
>>>> motivate not supporting VACUUM, but it might be a useful simplification to
>>>> keep in mind.  What about ANALYZE support; how important is the ability to
>>>> collect statistics on temporary tables?  Again, I tentatively expect to
>>>> support it regardless of the answer.
>>>
>>> I think it's probably pretty important to support VACUUM, because even
>>> ignoring wraparound considerations, not vacuuming tends to cause
>>> performance to suck.  I think ANALYZE is less important for the
>>> reasons stated above.
>>
>> ANALYZE is essential for temp tables in many cases... not sure what
>> the "reasons stated above" were, I can't resolve that reference.
>
> My theory is that users of a global temp table will have
> similar-enough usage patterns that a set of statistics that is good
> enough for one user will be good enough for all of them.  That might
> not be true in all cases, but I think it will simplify things quite a
> bit to assume it true for purposes of an initial implementation.  And
> as I noted, in some cases it might be a clear improvement: right now,
> after creating a temp table, you've got to analyze it or you'll just
> get the default statistics, which figure to be terrible.  Inheriting
> the statistics left over from the last guy's analyze figures to be
> significantly superior.

Oh, we're talking about different things, and I'm slightly confused.

Yes, we need to support ANALYZE; what we might not need to support, at
least initially, is every user of a global temp table having their own
SEPARATE copy of the table statistics.

--
Robert Haas
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