Excerpts from Alvaro Herrera's message of mar ene 24 15:47:16 -0300 2012:

> Need more code changes for the following:

> * export FOR KEY UPDATE lock mode in SQL

While doing this, I realized that there's an open item here regarding a
transaction that locks a tuple, and then in an aborted savepoint deletes
it.  As things stand, what happens is that the original tuple lock is
forgotten entirely, which was one of the things I wanted to fix (and in
fact is fixed for all other cases AFAICS).  So what we need is to be
able to store a MultiXactId that includes a member for KeyUpdate locks,
which will represent an UPDATE that touches key columns as well as
DELETEs.  That closes the hole.  However, the problem with this is that
we have no more bits left in the flag bitmask, which is another concern
you had raised.  I chose the easy way out and added a full byte of flags
per transaction.

This means that we now have 1636 xacts per members page rather than
1900+, but I'm not too concerned about that.  (We could cut back to 4
flag bits per xact -- but then, having some room for future growth is
probably a good plan anyway).

So DELETEs can also create multis.  I'm going to submit an updated patch

Álvaro Herrera <alvhe...@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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