Date: Fri, 23 Dec 2016 16:48:59 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Pavan Deolasee <pavan.deolasee@gmail.com>
Cc: Haribabu Kommi <kommi.haribabu@gmail.com>,
	Tomas Vondra <tomas.vondra@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Patch: Write Amplification Reduction Method (WARM)
Message-ID: <20161223194859.ryiopmds22zr5ts3@alvherre.pgsql>
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Alvaro Herrera wrote:

> With your WARM and my indirect indexes, plus the additions for for-key
> locks, plus identity columns, there is no longer a real expectation that
> we can exit early from the function.  In your patch, as well as mine,
> there is a semblance of optimization that tries to avoid computing the
> updated_attrs output bitmapset if the pointer is not passed in, but it's
> effectively pointless because the only interesting use case is from
> ExecUpdate() which always activates the feature.  Can we just agree to
> drop that?

I think the only case that gets worse is the path that does
simple_heap_update, which is used for DDL.  I would be very surprised if
a change there is noticeable, when compared to the rest of the stuff
that goes on for DDL commands.

Now, after saying that, I think that a table with a very large number of
columns is going to be affected by this.  But we don't really need to
compute the output bits for every single column -- we only care about
those that are covered by some index.  So we should pass an input
bitmapset comprising all such columns, and the output bitmapset only
considers those columns, and ignores columns not indexed.  My patch for
indirect indexes already does something similar (though it passes a
bitmapset of columns indexed by indirect indexes only, so it needs a
tweak there.)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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