Heikki Linnakangas napsal(a):
On 12/06/10 17:18, Pavel Baros wrote:
I am curious how could I solve the problem:
During refreshing I would like to know, if MV is stale or fresh? And I
had an idea:
In fact, MV need to know if its last refresh (transaction id) is older
than any INSERT, UPDATE, DELETE transaction launched against source
tables. So if MV has information about last (highest) xmin in source
tables, it could simply compare its own xmin to xmins (xmax for deleted
rows) from source tables and decide, if is stale or fresh.
Whole realization could look like this:
1. Make new column in pg_class (or somewhere in pg_stat* ?):
pg_class.rellastxid (of type xid)
2. After each INSERT, UPDATE, DELETE statement (transaction)
pg_class.rellastxid would be updated. That should not be time- or
memory- consuming (not so much) since pg_class is cached, I guess.
rellastxid would have to be updated at every insert/update/delete. It
would become a big bottleneck. That's not going to work.
Why do you need to know if a MV is stale?
Sorry I did not mention it. If we knew MV is fresh, there is no neeed to
refresh MV and so it would prevent useless rebuilding of MV. So I
thought there is room for saving some work.
Anyway, I realized, this idea do not cover all the cases how to find out
MV is stale or fresh. For example, when updating a row of source table
of MV, that do not participate in MV, in that case refreshing of MV
would be useless too.
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