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https://github.com/df7cb/pg_dirtyread

Read dead but unvacuumed tuples from a PostgreSQL relation
https://github.com/df7cb/pg_dirtyread

extension pg-dirtyread postgresql uncommitted

Last synced: 21 days ago
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Read dead but unvacuumed tuples from a PostgreSQL relation

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README 

        
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:



  ```sql

    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:



  ```sql

    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.



  ```sql

    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.



  ```sql

    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

-------



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.



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.