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 |
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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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