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Commit 1f92630

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Jan WieckJan Wieck
Jan Wieck
authored and
Jan Wieck
committed
Added the Skytools extended transaction ID module to contrib as discussed
on CORE previously. This module offers transaction ID's containing the original XID and the transaction epoch as a bigint value to the user level. It also provides a special txid_snapshot data type that contains an entire transactions visibility snapshot information, which is useful to determine if a particular txid was visible to a transaction or not. The module has been tested by porting Slony-I from using its original xxid data type. Jan
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‎contrib/Makefile

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# $PostgreSQL: pgsql/contrib/Makefile,v 1.78 2007/08/21 01:12:28 tgl Exp $
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# $PostgreSQL: pgsql/contrib/Makefile,v 1.79 2007/10/07 23:32:19 wieck Exp $
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subdir = contrib
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top_builddir = ..
@@ -30,6 +30,7 @@ WANTED_DIRS = \
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pgstattuple \
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seg \
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spi \
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txid \
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tablefunc \
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vacuumlo
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‎contrib/txid/Makefile

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MODULES = txid
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DATA_built = txid.sql
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DATA = uninstall_txid.sql
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DOCS = README.txid
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REGRESS = txid
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ifdef USE_PGXS
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PG_CONFIG = pg_config
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PGXS := $(shell $(PG_CONFIG) --pgxs)
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include $(PGXS)
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else
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subdir = contrib/txid
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top_builddir = ../..
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include $(top_builddir)/src/Makefile.global
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include $(top_srcdir)/contrib/contrib-global.mk
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endif
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test: install
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$(MAKE) installcheck || { less regression.diffs; exit 1; }
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ack:
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cp results/* expected/
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‎contrib/txid/README.txid

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

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-- init
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\set ECHO none
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-- i/o
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select '12:13:'::txid_snapshot;
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txid_snapshot
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---------------
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12:13:
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(1 row)
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select '12:13:1,2'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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-- errors
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select '31:12:'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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select '0:1:'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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select '12:13:0'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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select '12:16:14,13'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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select '12:16:14,14'::txid_snapshot;
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ERROR: illegal txid_snapshot input format
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create table snapshot_test (
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nr integer,
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snap txid_snapshot
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);
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insert into snapshot_test values (1, '12:13:');
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insert into snapshot_test values (2, '12:20:13,15,18');
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insert into snapshot_test values (3, '100001:100009:100005,100007,100008');
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insert into snapshot_test values (4, '100:150:101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131');
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select snap from snapshot_test order by nr;
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snap
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-------------------------------------------------------------------------------------------------------------------------------------
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12:13:
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12:20:13,15,18
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100001:100009:100005,100007,100008
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100:150:101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131
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(4 rows)
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select txid_snapshot_xmin(snap),
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txid_snapshot_xmax(snap),
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txid_snapshot_xip(snap)
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from snapshot_test order by nr;
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txid_snapshot_xmin | txid_snapshot_xmax | txid_snapshot_xip
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--------------------+--------------------+-------------------
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12 | 20 | 13
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12 | 20 | 15
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12 | 20 | 18
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100001 | 100009 | 100005
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100001 | 100009 | 100007
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100001 | 100009 | 100008
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100 | 150 | 101
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100 | 150 | 102
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100 | 150 | 103
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100 | 150 | 104
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100 | 150 | 105
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100 | 150 | 106
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100 | 150 | 107
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100 | 150 | 108
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100 | 150 | 109
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100 | 150 | 110
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100 | 150 | 111
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100 | 150 | 112
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100 | 150 | 113
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100 | 150 | 114
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100 | 150 | 115
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100 | 150 | 116
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100 | 150 | 117
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100 | 150 | 118
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100 | 150 | 119
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100 | 150 | 120
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100 | 150 | 121
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100 | 150 | 122
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100 | 150 | 123
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100 | 150 | 124
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100 | 150 | 125
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100 | 150 | 126
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100 | 150 | 127
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100 | 150 | 128
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100 | 150 | 129
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100 | 150 | 130
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100 | 150 | 131
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(37 rows)
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select id, txid_visible_in_snapshot(id, snap)
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from snapshot_test, generate_series(11, 21) id
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where nr = 2;
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id | txid_visible_in_snapshot
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----+--------------------------
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11 | t
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12 | t
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13 | f
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14 | t
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15 | f
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16 | t
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17 | t
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18 | f
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19 | t
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20 | f
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21 | f
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(11 rows)
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-- test bsearch
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select id, txid_visible_in_snapshot(id, snap)
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from snapshot_test, generate_series(90, 160) id
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where nr = 4;
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id | txid_visible_in_snapshot
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-----+--------------------------
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90 | t
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91 | t
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92 | t
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93 | t
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94 | t
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95 | t
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96 | t
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97 | t
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98 | t
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99 | t
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100 | t
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101 | f
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102 | f
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103 | f
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104 | f
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105 | f
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106 | f
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107 | f
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108 | f
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109 | f
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110 | f
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111 | f
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112 | f
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113 | f
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114 | f
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115 | f
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116 | f
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117 | f
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118 | f
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119 | f
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120 | f
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121 | f
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122 | f
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123 | f
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124 | f
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125 | f
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126 | f
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127 | f
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128 | f
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129 | f
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130 | f
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131 | f
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132 | t
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133 | t
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134 | t
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135 | t
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136 | t
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137 | t
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138 | t
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139 | t
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140 | t
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141 | t
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142 | t
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143 | t
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144 | t
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145 | t
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146 | t
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147 | t
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148 | t
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149 | t
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150 | f
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151 | f
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152 | f
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153 | f
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154 | f
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155 | f
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156 | f
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157 | f
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158 | f
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159 | f
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160 | f
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(71 rows)
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-- test current values also
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select txid_current() >= txid_snapshot_xmin(txid_current_snapshot());
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?column?
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----------
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t
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(1 row)
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/* due to lazy xid alloc in 8.3 those are different in 8.2 and 8.3
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select txid_current() < txid_snapshot_xmax(txid_current_snapshot());
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select txid_visible_in_snapshot(txid_current(), txid_current_snapshot());
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*/
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-- test 64bitness
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select txid_snapshot '1000100010001000:1000100010001100:1000100010001012,1000100010001013';
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txid_snapshot
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---------------------------------------------------------------------
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1000100010001000:1000100010001100:1000100010001012,1000100010001013
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(1 row)
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select txid_visible_in_snapshot('1000100010001012', '1000100010001000:1000100010001100:1000100010001012,1000100010001013');
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txid_visible_in_snapshot
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--------------------------
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f
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(1 row)
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select txid_visible_in_snapshot('1000100010001015', '1000100010001000:1000100010001100:1000100010001012,1000100010001013');
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txid_visible_in_snapshot
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--------------------------
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t
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(1 row)
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