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Re: index-only scans vs. Hot Standby, round two

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: index-only scans vs. Hot Standby, round two
Date: 2012-05-02 12:41:02
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Lists: pgsql-hackers
On Thu, Apr 26, 2012 at 8:03 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> So, as a first step, I've committed a patch that just throws a hard
> conflict.  I think we probably want to optimize this further, and I'm
> going to work investigate that next.  But it seemed productive to get
> this much out of the way first, so I did.

I've been thinking about this some more.  What's worrying me is that a
visibility conflict, however we implement it, could be *worse* from
the user's point of view than just killing the query.  After all,
there's a reasonable likelihood that a single visibility map page
covers the whole relation (or all the blocks that the user is
interested in), so any sort of conflict is basically going to turn the
index-only scan into an index-scan plus some extra overhead.  And if
the planner had known that the user was going to get an index-only
scan rather than just a plain index scan, it might well have picked
some other plan in the first place.

Another problem is that, if we add a test for visibility conflicts
into visibilitymap_test(), I'm afraid we're going to drive up the cost
of that function very significantly.  Previous testing suggests that
that efficiency or lack thereof of that function is already a
performance problem for index-only scans, which kinda makes me not
that excited about adding another branch in there somewhere (and even
less excited about any proposed implementation that would add an
lwlock acquire/release or similar).

So on further reflection I'm thinking it may be best just to stick
with a hard conflict for now and see what feedback we get from beta

Robert Haas
The Enterprise PostgreSQL Company

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