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Re: [HACKERS] It sorta works, but I'm confused about locking

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] It sorta works, but I'm confused about locking
Date: 1998-10-02 00:25:15
Message-ID: 29210.907287915@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> writes:
> I recommend you check out what is currently done properly, and fix the
> ones that are incorrect.

Well, yes.  The question was how to tell which is which :-)

> I can imagine some cases where you would want to get a lock and keep it
> until the end of the transaction, and other times when you would want to
> release it before transaction end.

I guess I'm not understanding something.  How can it ever be correct
practice to release a lock before transaction end?  For example, if I
write some changes in a table, and then release the lock, wouldn't that
allow other backends to see the not-yet-committed changes?  What if I
then abort my transaction?  Now the other backends have acted on
information they should never have seen at all.

Releasing a read lock strikes me as just as dangerous but for more
subtle reasons --- once you have read a table, what you have read
ought to look the same until the end of your transaction.

Since there is an unset-write-lock function, I assume it must have
valid uses, but I don't see what they are.

>> Is there a bug here, or is there some special definition of user access
>> to a system table that means the select isn't acquiring a read lock?
>> Selects and updates on ordinary user tables seem to interlock fine...

> Select certainly should be locking.  Something is wrong, but I am not
> sure what.  If you want me to check into it, let me know.

Please.  (Note that I saw this with my revised version of async.c;
I believe you will see the same behavior with the currently-checked-in
code, but do not have the time to rebuild that version to make sure.)

			regards, tom lane

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Subject: Re: [HACKERS] Proper cleanup at backend exit
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