Re: the XID question
От | Greg Smith |
---|---|
Тема | Re: the XID question |
Дата | |
Msg-id | 4D3769DB.4090804@2ndquadrant.com обсуждение исходный текст |
Ответ на | Re: the XID question ("Kevin Grittner" <Kevin.Grittner@wicourts.gov>) |
Список | pgsql-performance |
Kevin Grittner wrote: > Or just test it in psql. BEGIN, run your query, look at pg_locks. > If an xid has been assigned, you'll see it there in the > transactionid column. You can easily satisfy yourself which > statements grab an xid... That's a good way to double-check exactly what's happening, but it's not even that hard: gsmith=# select txid_current(); txid_current | 696 gsmith=# select 1; ?column? | 1 gsmith=# select 1; ?column? | 1 gsmith=# select txid_current(); txid_current | 697 Calling txid_current bumps the number up, but if you account for that you can see whether the thing(s) in the middle grabbed a real txid by whether the count increased by 1 or more than that. So here's what one that did get a real xid looks like: gsmith=# select txid_current(); txid_current | 702 gsmith=# insert into t(i) values(1); INSERT 0 1 gsmith=# select txid_current(); txid_current | 704 That proves the INSERT in the middle was assigned one. The commit message that added this feature to 8.3 has a good quick intro to what changed from earlier revs: http://archives.postgresql.org/pgsql-committers/2007-09/msg00026.php Don't have to actually read the source to learn a bit more, because it's actually documented! Mechanics are described at pgsql/src/backend/access/transam/README ; you need to know a bit more about subtransactions to follow all of it, but it gets the general idea across regardless: = Transaction and Subtransaction Numbering = Transactions and subtransactions are assigned permanent XIDs only when/if they first do something that requires one --- typically, insert/update/delete a tuple, though there are a few other places that need an XID assigned. If a subtransaction requires an XID, we always first assign one to its parent. This maintains the invariant that child transactions have XIDs later than their parents, which is assumed in a number of places. The subsidiary actions of obtaining a lock on the XID and and entering it into pg_subtrans and PG_PROC are done at the time it is assigned. A transaction that has no XID still needs to be identified for various purposes, notably holding locks. For this purpose we assign a "virtual transaction ID" or VXID to each top-level transaction. VXIDs are formed from two fields, the backendID and a backend-local counter; this arrangement allows assignment of a new VXID at transaction start without any contention for shared memory. To ensure that a VXID isn't re-used too soon after backend exit, we store the last local counter value into shared memory at backend exit, and initialize it from the previous value for the same backendID slot at backend start. All these counters go back to zero at shared memory re-initialization, but that's OK because VXIDs never appear anywhere on-disk. Internally, a backend needs a way to identify subtransactions whether or not they have XIDs; but this need only lasts as long as the parent top transaction endures. Therefore, we have SubTransactionId, which is somewhat like CommandId in that it's generated from a counter that we reset at the start of each top transaction. The top-level transaction itself has SubTransactionId 1, and subtransactions have IDs 2 and up. (Zero is reserved for InvalidSubTransactionId.) Note that subtransactions do not have their own VXIDs; they use the parent top transaction's VXID. -- Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
В списке pgsql-performance по дате отправления: