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	id AA21775; Wed, 11 Mar 92 17:20:27 -0800
Date: Wed, 11 Mar 92 17:20:27 -0800
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From: joseash@bush.tamu.edu (Alfredo Sanchez)
Subject: Re: 2-phase locks and deadlocks
To: postgres@postgres.berkeley.edu
Sender: pg_adm@postgres.berkeley.edu



	i write:

	> when doing postquel queries within
	> a transaction, is there any fixed order
	> in which locks are acquired, or could it be
	> different depending upon the particular situation
	> when the transaction starts?

	you write:

	The locks are acquired on an as-needed basis.  That is if you scan
	a relation (e.g. during a retrieve) a read lock is acquired before the
	scan begins.  If you are appending tuples into a relation a write lock
	is acquired before the insert takes place.  So the order that locks
	are acquired depends on the order of operations in the xaction block.
	When you are inside such a block, acquired locks will not be released
	until you "end" it.  At the end of each and every xaction all locks
	for that xaction are released (complying with 2-pl protocol).

i'm not sure i'm getting it correctly.
if i have for example, 3 xactions like

      I                          II                         III

begin                     begin                            begin
   replace A (...)           replace B (...)                  replace A(...)
   replace B (...)        end                              end
   replace C (...)                       
end

possibly running concurrently. Assuming that II grabs B first and then III 
grabs A. if II releases B, will 'I' still wait for III to release
A, without grabbing B meanwhile?
in other words, will 'I' always acquire the locks in an A-B-C sequence?
sorry about the messy way to pose the question, i'm trying
to guarantee deadlock free xactions.
thanks in advance,

J Alfredo Sanchez H
-------------------
Hypertext Research Lab
Department of Computer Science
Texas A&M University