mirror of
https://git.postgresql.org/git/postgresql.git
synced 2024-09-11 03:43:22 +02:00
…
|
||
---|---|---|
.. | ||
expected | ||
sql | ||
Makefile | ||
README.txid | ||
txid.c | ||
txid.sql.in | ||
uninstall_txid.sql |
txid - export transaction id's to user level ============================================ The goal is to make PostgreSQL internal transaction ID and snapshot data usable externally. This allows very efficient queue implementation done inside database. [towrite: what snapshot means] The module defines type txid_snapshot and following functions: txid_current() returns int8 Current transaction ID. txid_current_snapshot() returns txid_snapshot Current snapshot. txid_snapshot_xmin( snap ) returns int8 Smallest TXID in snapshot. TXID's smaller than this are all visible in snapshot. txid_snapshot_xmax( snap ) returns int8 Largest TXID in snapshot. TXID's starting from this one are all invisible in snapshot. txid_snapshot_xip( snap ) setof int8 List of in-progress TXID's in snapshot, that are invisible. Values are between xmin and xmax. txid_visible_in_snapshot(id, snap) returns bool Is TXID visible in snapshot? Fetching events --------------- Lets say there is following event table: CREATE TABLE events ( ev_txid int8 not null default txid_current(), ev_data text ); CREATE INDEX ev_txid_idx ON events (ev_txid); Then event between 2 snapshots snap1 and snap2 can be fetched with followign query: SELECT ev_data FROM events WHERE ev_txid >= txid_snapshot_xmin(:snap1) AND ev_txid < txid_snapshot_xmax(:snap2) AND NOT txid_visible_in_snapshot(ev_txid, :snap1) AND txid_visible_in_snapshot(ev_txid, :snap2); This is the simplest query but it has problem if there are long transactions running - the txid_snapshot_xmin(snap1) will stay low and the range will get very large. This can be fixed by fetching only snap1.xmax ... snap1.xmax by range and fetching possible txids below snap1.xmax explicitly: SELECT ev_data FROM events WHERE ((ev_txid >= txid_snapshot_xmax(:snap1) AND ev_txid < txid_snapshot_xmax(:snap2)) OR (ev_txid IN (SELECT * FROM txid_snapshot_xip(:snap1)))) AND NOT txid_visible_in_snapshot(ev_txid, :snap1) AND txid_visible_in_snapshot(ev_txid, :snap2); Note that although the above queries work, the PostgreSQL fails to plan them correctly. For actual usage the values for txid_snapshot_xmin, txid_snapshot_xmax and txid_snapshot_xip should be filled in directly, only then will they use index. There are few more optimizations possible, like: - Picking out only TXIDs that were actually committed between snap1 and snap2. - Lowering the range from txid_snapshot_xmax(snap1) to decrease the list if TXIDs to be fetched separately. To see example code for that it's best to see pgq.batch_event_sql() function in Skytools. http://pgfoundry.org/projects/skytools/ Dumping and restoring data containing TXIDs. -------------------------------------------- [towrite: reason for epoch increase] You can look at current epoch with query: SELECT txid_current() >> 32 as epoch; So new epoch should be: SELECT (txid_current() >> 32) + 1 as newepoch; Epoch can be changed with pg_resetxlog command: pg_resetxlog -e NEWEPOCH DATADIR Database needs to be shut down for that moment.