pg_dirtyread

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.)

Building

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

Loading and Using

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 Test

Dropped Columns

The 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 │ World

System Columns

System 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 inserted

Authors

pg_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.

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
Neither the name OmniTI Computer Consulting, Inc. nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.