The pg_dirtyread extension provides the ability to read dead but unvacuumed rows from a relation. Supports PostgreSQL 9.2 and later. (On 9.2, at least 9.2.9 is required.)
To build pg_dirtyread, just do this:
make
make install
If you encounter an error such as:
make: pg_config: Command not found
Be sure that you have pg_config installed and in your path. If you used a
package management system such as RPM to install PostgreSQL, be sure that the
-devel package is also installed. If necessary tell the build process where
to find it:
make PG_CONFIG=/path/to/pg_config
make install PG_CONFIG=/path/to/pg_config
Once pg_dirtyread is built and installed, you can add it to a database. Loading pg_dirtyread is as simple as connecting to a database as a super user and running:
CREATE EXTENSION pg_dirtyread;
SELECT * FROM pg_dirtyread('tablename') AS t(col1 type1, col2 type2, ...);The pg_dirtyread() function returns RECORD, therefore it is necessary to
attach a table alias clause that describes the table schema. Columns are
matched by name, so it is possible to omit some columns in the alias, or
rearrange columns.
Example:
CREATE EXTENSION pg_dirtyread;
-- Create table and disable autovacuum
CREATE TABLE foo (bar bigint, baz text);
ALTER TABLE foo SET (
autovacuum_enabled = false, toast.autovacuum_enabled = false
);
INSERT INTO foo VALUES (1, 'Test'), (2, 'New Test');
DELETE FROM foo WHERE bar = 1;
SELECT * FROM pg_dirtyread('foo') as t(bar bigint, baz text);
bar │ baz
─────┼──────────
1 │ Test
2 │ New TestThe content of dropped columns can be retrieved as long as the table has not
been rewritten (e.g. via VACUUM FULL or CLUSTER). Use dropped_N to access
the Nth column, counting from 1. PostgreSQL deletes the type information of the
original column, so only a few sanity checks can be done if the correct type
was specified in the table alias; checked are type length, type alignment, type
modifier, and pass-by-value.
CREATE TABLE ab(a text, b text);
INSERT INTO ab VALUES ('Hello', 'World');
ALTER TABLE ab DROP COLUMN b;
DELETE FROM ab;
SELECT * FROM pg_dirtyread('ab') ab(a text, dropped_2 text);
a │ dropped_2
───────┼───────────
Hello │ WorldSystem columns such as xmax and ctid can be retrieved by including them in
the table alias attached to the pg_dirtyread() call. A special column dead of
type boolean is available to report dead rows (as by HeapTupleIsSurelyDead).
The dead column is not usable during recovery, i.e. most notably not on
standby servers. The oid column is only available in PostgreSQL version 11
and earlier.
SELECT * FROM pg_dirtyread('foo')
AS t(tableoid oid, ctid tid, xmin xid, xmax xid, cmin cid, cmax cid, dead boolean,
bar bigint, baz text);
tableoid │ ctid │ xmin │ xmax │ cmin │ cmax │ dead │ bar │ baz
──────────┼───────┼──────┼──────┼──────┼──────┼──────┼─────┼───────────────────
41823 │ (0,1) │ 1484 │ 1485 │ 0 │ 0 │ t │ 1 │ Delete
41823 │ (0,2) │ 1484 │ 0 │ 0 │ 0 │ f │ 2 │ Insert
41823 │ (0,3) │ 1484 │ 1486 │ 0 │ 0 │ t │ 3 │ Update
41823 │ (0,4) │ 1484 │ 1488 │ 0 │ 0 │ f │ 4 │ Not deleted
41823 │ (0,5) │ 1484 │ 1489 │ 1 │ 1 │ f │ 5 │ Not updated
41823 │ (0,6) │ 1486 │ 0 │ 0 │ 0 │ f │ 3 │ Updated
41823 │ (0,7) │ 1489 │ 0 │ 1 │ 1 │ t │ 5 │ Not quite updated
41823 │ (0,8) │ 1490 │ 0 │ 2 │ 2 │ t │ 6 │ Not insertedpg_dirtyread 1.0 was written by Phil Sorber in 2012. Christoph Berg added the ability to retrieve system columns in version 1.1, released 2017, and took over further maintenance.
Copyright (c) 1996-2024, PostgreSQL Global Development Group
Copyright (c) 2012, OmniTI Computer Consulting, Inc.
Portions Copyright (c) 1994, The Regents of the University of California
All rights reserved.
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