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serializable read only deferrable

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <pgsql-hackers(at)postgresql(dot)org>
Cc: <drkp(at)csail(dot)mit(dot)edu>
Subject: serializable read only deferrable
Date: 2010-12-05 15:11:35
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
I'm reviving the discussion on the subject topic because I just had
an epiphany which makes it seem simple to implement.  The concept of
this is that if you start a SERIALIZABLE READ ONLY transaction in an
SSI environment when certain conditions are true, it doesn't need to
acquire predicate locks or test for rw-conflicts.  This would be
particularly useful for pg_dump or large reports, as it would allow
them to read data which was guaranteed to be consistent with later
states of the database without risking serialization failure or
contributing to the failure of other transactions.  They should also
run a bit faster without the overhead of locking and checking.
Having completed the switch from a pair of rw-conflict pointers per
serializable transaction to a list of rw-conflicts, I'm working
through the more aggressive transaction clean-up strategies thereby
allowed in preparation for the graceful degradation code.  Along the
way, I noticed how easy it is to allow a READ ONLY transaction to opt
out of predicate locking and conflict detection when it starts with
no concurrent non READ ONLY transactions active, or even to remove
READ ONLY transactions from those activities when such a state is
reached during the execution of READ ONLY transactions; while
properly recognizing the *additional* conditions under which this
would be valid is rather painful.  (Those additional conditions being
that no concurrent non-read-only transaction may overlap a committed
non-read-only transaction which wrote data and committed before the
read-only transaction acquired its snapshot.)
The simple way to implement SERIALIZABLE READ ONLY DEFERRABLE under
SSI would be to have each non-read-only serializable transaction
acquire a heavyweight lock which can coexist with other locks at the
same level (SHARE looks good) on some common object and hold that for
the duration of the transaction, while a SERIALIZABLE READ ONLY
DEFERRABLE transaction would need to acquire a conflicting lock
(EXCLUSIVE looks good) before it could acquire a snapshot, and
release the lock immediately after acquiring the snapshot.
For these purposes, it appears that advisory locks could work, as
long as the lock release does not wait for the end of the transaction
(which it doesn't, if I'm reading the docs right) and as long as I
can pick a lock ID which won't conflict with other uses.  That latter
part is the only iffy aspect of the whole thing that I can see.  Of
course, I could add a third lock method, but that seems like overkill
to be able to get one single lock.
Since I'm already allowing a transaction to opt out of predicate
locking and conflict detection if there are no non-read-only
transactions active when it acquires its snapshot, the work needed
within the SSI code is pretty trivial; it's all in adding the
DEFERRABLE word as a non-standard extension to SET TRANSACTION et al,
and finding a heavyweight lock to use.


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