1+ txid - export transaction IDs to user level
2+ ===========================================
13
2- txid - export transaction id's to user level
3- ============================================
4-
5- The goal is to make PostgreSQL internal transaction ID and snapshot
4+ The goal is to make PostgreSQL's internal transaction ID and snapshot
65data usable externally. This allows very efficient queue
76implementation done inside database.
87
@@ -32,7 +31,7 @@ txid_snapshot_xmax( snap ) returns int8
3231txid_snapshot_xip( snap ) setof int8
3332
3433 List of in-progress TXID's in snapshot, that are invisible.
35- Values are between xmin and xmax.
34+ Values are between xmin(inclusive) and xmax (exclusive) .
3635
3736txid_visible_in_snapshot(id, snap) returns bool
3837
@@ -73,8 +72,8 @@ fetching possible txids below snap1.xmax explicitly:
7372 AND NOT txid_visible_in_snapshot(ev_txid, :snap1)
7473 AND txid_visible_in_snapshot(ev_txid, :snap2);
7574
76- Note that although the above queries work,the PostgreSQL fails to
77- plan themcorrectly . For actual usage the values for txid_snapshot_xmin,
75+ Note that although the above queries work, PostgreSQL fails to
76+ plan themefficiently . For actual usage the values for txid_snapshot_xmin,
7877txid_snapshot_xmax and txid_snapshot_xip should be filled in directly,
7978only then will they use index.
8079
@@ -92,20 +91,16 @@ To see example code for that it's best to see pgq.batch_event_sql() function in
9291Dumping and restoring data containing TXIDs.
9392--------------------------------------------
9493
95- [towrite: reason for epoch increase]
96-
97- You can look at current epoch with query:
98-
99- SELECT txid_current() >> 32 as epoch;
94+ When reloading TXID data you will typically want to be sure that the current
95+ XID counter is beyond the reloaded data. The easiest way to do this is to
96+ increase the XID epoch to beyond the largest one in the input data.
10097
101- So new epochshould be :
98+ You can look at current epochwith queries such as :
10299
103- SELECT(txid_current( ) >> 32) + 1 as newepoch ;
100+ SELECTMAX(txid ) >> 32 as epoch FROM ... ;
104101
105102Epoch can be changed with pg_resetxlog command:
106103
107104 pg_resetxlog -e NEWEPOCH DATADIR
108105
109106Database needs to be shut down for that moment.
110-
111-