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List:       postgresql-general
Subject:    Re: [GENERAL] Weird behavior with "sensitive" cursors.
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2009-09-30 22:29:26
Message-ID: 8262.1254349766 () sss ! pgh ! pa ! us
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Alvaro Herrera <alvherre@commandprompt.com> writes:
> Interesting.  If I create an non-unique index on the table before
> declaring the cursor, FETCH throws an error:

> alvherre=# fetch all from c1;
> ERROR:  attempted to lock invisible tuple

I get that in 8.4 and HEAD even without any index, just trying the given
case.  It looks to me like this is a bug in the new snapshot management.
The cursor is using CurrentSnapshot directly --- it does not have a
private copy --- and therefore when CommandCounterIncrement happens
it affects what the cursor can "see".  The cursor should not be able
to "see" any tuples created after it was created.

I think we need to ensure that when a cursor is created, it obtains a
private copy of the current snapshot ... but I'm not sure where that
ought to happen.  Thoughts?

			regards, tom lane

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