Hi, On 2015-02-02 11:15:22 +0100, Pavel Stehule wrote: > Six years ago we did discuss about global temporary tables - persistent > schema, ephemeral data. > > http://postgresql.nabble.com/idea-global-temp-tables-td2007217.html > > I am thinking so some reasons why implement this feature are valid: > > * we can get some performance benefit against current temp tables - less > the catalogue bloating, > > * we can simplify a static validation of plpgsql functions when temp tables > are used, > more global temp tables are little bit comfortable for developers, > > * we can simplify migration from some other databases, where global temp > tables are default.
I agree that the feature would be interesting. > 2. Implementation > > I see three possible ways how to implement it: > > 2.a - using on demand created temp tables - most simple solution, but > doesn't help with catalogue bloating
Yea, that's no good. > 2.b - using unlogged tables for holding statistics, relfilenode, and all > necessary data
I can't follow why that'd achieve anything?
1. Main catalogue will be stable.
2. There is not necessary to implement new storage and it can helps with transaction support.
> 3.c - store ephemeral metadata only in memory without MVCC
I think that's not an option. That'd end up being a massive amount of duplication at a low rate of functionality.
I don't plan to implement a storage - I expect only few functions for store/read data from session memory context
I think it's more realistic way to implement is to have a separate 'relpersistence' setting for global temp tables. The first access to such one in a session (or xact if truncate on commit) copies the table from the _init fork. By having the backend id in all filenames (besides the init fork) they're unique between sessions.
If I understand well, it is similar to my fast implementation from 2008. It works partially, because it doesn't solve other (session) property - like relpages, reltuples and related data from pg_statistics
Or something roughly like that.
Greetings,
Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
