Re: Measuring relation free space

On 11/06/2011 11:55 PM, Mark Kirkwood wrote:
> I am guessing (at this point very much guessing) that pg_freespace map
> may return its data faster, as pageinspect is gonna have to grovel
> through all the pages for itself (whereas pg_freespacemap relies on
> using info from the ... free space map fork).

I started with pageinspect because I wasn't sure if other methods would
be as accurate. For example, I wasn't sure until just before submission
that only the free space and size of the relation are needed to get a
useful measure here; at one point I was considering some other things
too. I've ruled them out as unnecessary as far as I can tell, but I
can't claim my tests are exhaustive. Some review confirmation that this
measure is useful for other people is one thing I was hoping for
feedback on, as one thing to consider in addition to the actual
implementation.

If this measurement is the only one needed, than as I said at the start
of the thread here it might easily be implemented to run just against
the free space map instead. I'm thinking of what's been sent so far as
a UI with matching output it should produce. If it's possible to get
the same numbers faster, exactly how to implement the function under the
hood is easy enough to change. Jaime already has a new version in
development that adds a ring buffer for example.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
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