Re: Using ALTER TABLESPACE in pg_dump

At 05:41 AM 21/10/2004, Robert Treat wrote:

>I've been thinking of an alternative solution that sounds very similar
>to this. The idea is to output the CREATE TABLESPACE commands inside
>pg_dump....
>...
>1) tablespace already exists
>to fail, but since we no longer stop on error during restore,

<soapbox>
A fact I positively loath! Relying on the 'bluder-on-regardless' approach 
is not something I'd like to enshrine.
</soapbox>


>2) if you have to restore on a machine with a different disk layout,
>give pg_restore a --override-tablespace command, which would substitute
>pg_default tablespace into the creation command of any tablespaces that
>get passed in. The bonus is that we would only have to parse on one
>specific command rather than worry about parsing several different
>commands. ...

I'm still inclined to avoid any parsing if at all possible. We should hit 
the code that generates the definitions (90% in pg_dump) and turn the 
definitions into more intelligent templates.



>... I was thinking that you could
>create two logical tablespaces on the same physical directory.

This is basically the virtual/fake tablespace idea.


Sometimes I think it is worth stepping back from a problem and ask what 
would we do if we had a clean slate, then use that to inform our current 
set of decisions. I'd be very interested in other people's ideas, but my 
thinking is:

- we might not have a tablespace clause inside object definitions; we could 
add a STORE TABLE xxx IN   TABLESPACE ts statement. Not really practical 
now; but worth bearing in mind as an approach for future non-standard syntax.
- we'd have a full set of ALTER xxx SET TABLESPACE commands which moved 
relevant data etc. We have most of them.
- pg_dump would issue the alter commands after creating the object; OK, it 
moves files, not very efficient, but using a 'SET MAGIC_TABLESPACE_VAR' has 
previously been discounted as a solution.

*If* you accept this as a good approach in an ideal world, then I think we 
need to ask ourselves if we should implement the remaining ALTER commands 
in 8.0 and be done with it.

The other solutions: magic-tablespace-var, virtual-tablespaces...all seem 
to add clunky functionality that will only be used in pg_dump. If we're 
going to add something, I'd prefer not to add clutter.




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