Re: PATCH: default_index_tablespace

On Wed, Apr 15, 2015 at 07:12:11PM -0400, Tom Lane wrote:
> jltallon@adv-solutions.net writes:
> > This small patch implements a new GUC (default_index_tablespace) plus 
> > supporting code.
> > Originated from a customer request, the feature intends to make 
> > creation of indexes on SSD-backed tablespaces easy and convenient 
> > (almost transparent) for users: the DBA can just set it and indexes will 
> > be placed in the specified tablespace --as opposed to the same 
> > tablespace where the referenced table is-- without having to specify it 
> > every time.
> 
> I'm afraid this idea is a nonstarter, because it will break existing
> applications, and in particular existing pg_dump output files, which
> expect to be able to determine an index's tablespace by setting
> "default_tablespace".  (It is *not* adequate that the code falls back
> to "default_tablespace" if the new GUC is unset; if it is set, you've
> still broken pg_dump.)  The incremental value, if indeed there is any,
> of being able to control index positioning this way seems unlikely to
> justify a backwards-compatibility break of such magnitude.

I can see why someone would want this because random I/O, which is
frequent for indexes, is much faster on SSD than magnetic disks. 
(Sequential I/O is more similar for the two.)

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
 + Everyone has their own god. +