Re: [Patch] Temporary tables that do not bloat pg_catalog (a.k.a fast temp tables)

On 07/30/2016 04:47 PM, Tom Lane wrote:
> Pavel Stehule <pavel.stehule@gmail.com> writes:
>> But there are some patterns used with work with temp tables,that should not
>> working, and we would to decide if we prepare workaround or not.
>
>> -- problematic pattern (old code)
>> IF NOT EXISTS(SELECT * FROM pg_class WHERE ....) THEN
>>   CREATE TEMP TABLE xxx()
>> ELSE
>>   TRUNCATE TABLE xxx;
>> END IF;
>
>> -- modern patter (new code)
>> BEGIN
>>   TRUNCATE TABLE xxx;
>> EXCEPTION WHEN ..... THEN
>>   CREATE TEMP TABLE(...)
>> END;
>
> If the former stops working, that's a sufficient reason to reject the
> patch: it hasn't been thought through carefully enough.  The key reason
> why I don't think that's negotiable is that if there aren't (apparently)
> catalog entries corresponding to the temp tables, that will almost
> certainly break many things in the backend and third-party extensions,
> not only user code patterns like this one.  We'd constantly be fielding
> bug reports that "feature X doesn't work with temp tables anymore".
>

Agreed - breaking internal features for temporary tables is not 
acceptable. I was thinking more about external code messing with 
catalogs, but on second thought we probably need to keep the records in 
pg_class anyway.
>
> In short, I think that the way to make something like this work is
> to figure out how to have "virtual" catalog rows describing a temp
> table. Or maybe to partition the catalogs so that vacuuming away
> temp-table rows is easier/cheaper than today.
>

Yeah, and I think the patch tries to do that, although in a rather 
invasive / unprincipled way. But this will only work for the current 
behavior (i.e. mostly what SQL standard means by LOCAL). For GLOBAL 
temporary tables I think we need to keep physical catalog row, and only 
override the storage filename.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services