Hi,
last week there was a discussion on linkedin related to port Oracle's application to Postgres.
I am sure so lot of usage of temporary tables in application is useless, based on long history of ported applications - Sybase (MSSQL) -> Oracle -> Postgres, but still global temporary tables are interesting feature - and impossibility to use GTT is a real problem for lot of users.
One of the issues of this port are probably temporary tables. It is probably a common issue - because PostgreSQL doesn't support global temporary tables and any workarounds have a significant problem with bloating of some system catalog tables - pg_attribute, pg_class, pg_depends, pg_shdepends.
The implementation has two parts - one can be "simple" - using a local storage for a persistent table.
Second is almost impossible - storing some metadata that cannot be shared - like relpages, reltuples, pg_statistic. We also want to support some views like pg_stats for global temp tables too, and if possibly without bigger changes.
Some years ago there was a some implementations based on using some memory caches. It doesn't work well, because Postgres has not concept of session persistent caches of catalog data, that should live across cache invalidation signal.
I think so this problem can be reduced just on implementation of pg_statistic table. If we can support GTT for pg_statistic we can support GTT generally.
pg_statistic can be (in future) partitioned table - one partition can for common tables, one partition can be global temporary tables. The partition for global temporary tables can be GTT by self. There can be a GTT partition for currently used local temporary tables too (this pattern can fix a bloating related to usage of local temporary tables).
I am not sure if proposed design is implementable - it requires partitioning of system tables on some very low level.
Has somebody some ideas to this topic?
Regards
Pavel
