Re: Improving free space usage (was: Reducing relation locking - Mailing list pgsql-hackers

Ühel kenal päeval, N, 2005-12-08 kell 12:57, kirjutas Jim C. Nasby:
> On Thu, Dec 08, 2005 at 11:58:50AM +0200, Hannu Krosing wrote:

> > > > The problem with separate list is that it can be huge. For example on a
> > > > table with 200 inserts/updates per second an index build lasting 6 hours
> > > > would accumulate total on 6*3600*200 = 4320000 new tuples.
> > > 
> > > Sure, but it's unlikely that such a table would be very wide, so 4.3M
> > > tuples would probably only amount to a few hundred MB of data. It's also
> > > possible that this list could be vacuumed by whatever the regular vacuum
> > > process is for the table.
> > 
> > I think that keeping such list as part the table at well defined
> > location (like pages from N to M) is the best strategy, as it will
> > automatically make all new tuples available to parallel processes and
> > avoids both duplicate storage as well as the the need for changing
> > insert/update code.
> 
> There's one thing I hate about that idea though: good luck trying to
> move those tuples somewhere else after the index build is done and you
> now want to shrink the table back down to a more normal size. 

I feel your pain.

To solve similar problem I have been forced to write scripts that do
updates of pk_column=pk_column until the tuple moves to another page as
shown by ctid . Not a sensible thing to do (do a lot of updates to
*increase* performance), but necessary nevertheless considering  current
postgres behaviour. 

> If we had
> a better way to do that it would be much more palatable, but right now
> on a heavily updated table this would result in a lot of bloat.

Actually any long transaction would do that.

> Along those lines, I've wondered if it makes sense to add more
> flexibility in how free space is reclaimed in a table. One obvious
> possibility is to have a strategy where new tuples will always look to
> the FSM for space (instead of going into the current page if possible),
> and the FSM will always hand out the earliest page in the table it has.
> This mode would have the effect of moving tuples towards the front of
> the table, allowing for space reclamation. A variation might be that
> this mode will not effect tuples that are generated as part of an UPDATE
> and are in the first x% of the table, since it doesn't make sense to
> move a tuple from page 2 to page 1 in a 1000 page table.

This % could be depending on some "fill factor" of the table, aiming not
to move tuples, that would end up in the final volume of a balance
table, which, in case of heavily updated table, would probably be 2 to 3
times the volume of densely populated table.

> Another possibility is to always go to the FSM and to have the FSM hand
> back the page that is closest to the new tuple according to a certain
> index. This would allow for ALTER TABLE CLUSTER to be much more
> self-maintaining. The downside is that you'd have to do a lookup on that
> index, but presumably if the index is important enough to cluster on
> then it should be well-cached. There's probably some other tweaks that
> could be done as well to make this more performant.

Yes, I agree on all your points about better placement of new tuples,
all they would be useful indeed.

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Hannu