From: | "Mike C" <smith(dot)not(dot)western(at)gmail(dot)com> |
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To: | pgsql-general(at)postgresql(dot)org |
Subject: | Postgres 8.3 HOT and non-persistent xids |
Date: | 2007-12-19 22:14:33 |
Message-ID: | bd0eabd0712191414o64aa5193w5fe57dff622c178c@mail.gmail.com |
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Lists: | pgsql-general |
Hi,
Can someone clarify HOT updates for me (and perhaps put more detail
into the docs?). Is this statement correct: the HOT technique is used
on *any* table so long as no indexed column is affected.
create table T (A int, B int);
create index TB on T (B);
insert into T (A,B) Values (1,2);
So if I do an update that is identical to the existing row, nothing changes?
update T set A=1, B=2 where A=1;
If I change the non-indexed field, A, then HOT applies and no new tuple needed?
update T set A=2, B=2 where A=1;
If I change the indexed field, B, then HOT doesn't apply and a new
tuple is needed?
update T set A=2,B=3 where A=2;
Is that correct?
Actually, what actually happens when you get an update with redundant
information, e.g.
update T set A=2,B=4 where A=2;
The value of A hasn't changed, does postgres still write the value?
And for non-persistent transaction ids, the documentation says that
this is for read-only transactions. What defines a read-only
transaction for this purpose? Does postgres check to see if a SELECT
includes e.g. a sequence change via nextval? If I mark the transaction
as readonly using the PG JDBC driver, will that be sufficient?
Thank you,
Mike
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