On Thu, Apr 16, 2015 at 8:01 AM, Bruce Momjian <
bruce@momjian.us> wrote:
>
> 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.)
>
Another way to provide different default tablespace for index could be
to provide it at Database level. Have a new option INDEX_TABLESPACE
in Create Database command which can be used to create indexes
when not specified during Create Index command. This would also need
changes in pg_dump (like while dumping info about database) but on
initial look, it seems it can be done without much changes.