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gincostestimate

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: gincostestimate
Date: 2011-04-08 21:07:06
Message-ID: BANLkTinz2do3EqWO9RH66bY2FBOmL=uSoA@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Dear Hackers,

A gin index created on an initially empty table will never get used
until the table is vacuumed, which for a table with no update or
delete activity could be forever unless someone manually intervenes.


The problem is that numEntries in src/backend/utils/adt/selfuncs.c is
zero and eventually causes a division by zero and a cost estimate of
nan.

The code below does not save the day, because nTotalPages and
nEntryPages are 2 and 1 respectively when an index is created on an
empty table, or when an indexed table is truncated.

    if (ginStats.nTotalPages == 0 || ginStats.nEntryPages == 0)
    {
        numEntryPages = numPages;
        numDataPages = 0;
        numEntries = numTuples;     /* bogus, but no other info available */
    }

I don't know what the solution is.  Simply setting numEntries to 1 if
ginStats.nEntries zero solves this particular problem, but I don't
know what other consequences it might have.

Cheers,

Jeff

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