Re: relation ### modified while in use

At 10:45 23/10/00 -0400, Tom Lane wrote:
>Philip Warner <pjw(at)rhyme(dot)com(dot)au> writes:
>> Don't we have this ability? What about taking a RowShare lock on the
>> pg_class tuple whenever you read from the table; then requiring schema
>> updates take a RowExclusive lock on the pg_class tuple?
>
>How is that different from taking locks on the table itself?

Only slightly; one interpretation of a table lock is that it locks all of
the data in the table; and a lock on the pg_class row locks the metadata. I
must admit that I am having a little difficulty thinking of a case where
the distinction would be useful...

>In any case, we don't have the ability to hold multiple classes of locks
>on individual tuples, AFAIK. UPDATE and SELECT FOR UPDATE use a
>different mechanism that involves setting fields in the header of the
>affected tuple. There's no room there for more than one kind of lock;
>what's worse, checking and waiting for that lock is far slower than
>normal lock-manager operations.

So where do

SELECT FOR UPDATE IN ROW SHARE MODE
and
LOCK TABLE IN ROW EXCLUSIVE MODE statements.

fit in?

They *seem* to provide differing levels of row locking.

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