The pg_variables module provides functions to work with variables of various types. Created variables live only in the current user session. By default, created variables are not transactional (i.e. they are not affected by BEGIN, COMMIT or ROLLBACK statements). This, however, is customizable by argument is_transactional of pgv_set():

SELECT pgv_set('vars', 'int1', 101);
BEGIN;
SELECT pgv_set('vars', 'int2', 102);
ROLLBACK;

SELECT * FROM pgv_list() order by package, name;
 package | name | is_transactional
---------+------+------------------
 vars    | int1 | f
 vars    | int2 | f

But if variable created with flag is_transactional:

BEGIN;
SELECT pgv_set('vars', 'trans_int', 101, true);
SAVEPOINT sp1;
SELECT pgv_set('vars', 'trans_int', 102, true);
ROLLBACK TO sp1;
COMMIT;
SELECT pgv_get('vars', 'trans_int', NULL::int);
 pgv_get
---------
     101

You can aggregate variables into packages. This is done to be able to have variables with different names or to quickly remove the whole batch of variables. If the package becomes empty, it is automatically deleted.

This module available under the license similar to PostgreSQL.

Typical installation procedure may look like this:

$ cd pg_variables
$ make USE_PGXS=1
$ sudo make USE_PGXS=1 install
$ make USE_PGXS=1 installcheck
$ psql DB -c "CREATE EXTENSION pg_variables;"

The functions provided by the pg_variables module are shown in the tables below.

To use pgv_get() function required package and variable must exists. It is necessary to set variable with pgv_set() function to use pgv_get() function.

If a package does not exists you will get the following error:

SELECT pgv_get('vars', 'int1', NULL::int);
ERROR:  unrecognized package "vars"

If a variable does not exists you will get the following error:

SELECT pgv_get('vars', 'int1', NULL::int);
ERROR:  unrecognized variable "int1"

pgv_get() function check the variable type. If the variable type does not match with the function type the error will be raised:

SELECT pgv_get('vars', 'int1', NULL::text);
ERROR:  variable "int1" requires "integer" value

pgv_set arguments:

• package - name of the package, it will be created if it doesn't exist.
• name - name of the variable, it will be created if it doesn't exist. pgv_set fails if the variable already exists and its transactionality doesn't match is_transactional argument.
• value - new value for the variable. pgv_set fails if the variable already exists and its type doesn't match new value's type.
• is_transactional - transactionality of the newly created variable, by default it is false.

pgv_get arguments:

• package - name of the existing package. If the package doesn't exist result depends on strict argument: if it is false then pgv_get returns NULL otherwise it fails.
• name - name of the the existing variable. If the variable doesn't exist result depends on strict argument: if it is false then pgv_get returns NULL otherwise it fails.
• var_type - type of the existing variable. It is necessary to pass it to get correct return type.
• strict - pass false if pgv_get shouldn't raise an error if a variable or a package didn't created before, by default it is true.

The following functions are provided by the module to work with collections of record types.

To use pgv_update(), pgv_delete() and pgv_select() functions required package and variable must exists. Otherwise the error will be raised. It is necessary to set variable with pgv_insert() function to use these functions.

pgv_update(), pgv_delete() and pgv_select() functions check the variable type. If the variable type does not record type the error will be raised.

Note that pgv_stats() works only with the PostgreSQL 9.6 and newer.

It is easy to use functions to work with scalar and array variables:

SELECT pgv_set('vars', 'int1', 101);
SELECT pgv_set('vars', 'text1', 'text variable'::text);

SELECT pgv_get('vars', 'int1', NULL::int);
 pgv_get_int
-------------
         101

SELECT SELECT pgv_get('vars', 'text1', NULL::text);
    pgv_get
---------------
 text variable

SELECT pgv_set('vars', 'arr1', '{101,102}'::int[]);

SELECT pgv_get('vars', 'arr1', NULL::int[]);
  pgv_get
-----------
 {101,102}

Let's assume we have a tab table:

CREATE TABLE tab (id int, t varchar);
INSERT INTO tab VALUES (0, 'str00'), (1, 'str11');

Then you can use functions to work with record variables:

SELECT pgv_insert('vars', 'r1', tab) FROM tab;

SELECT pgv_select('vars', 'r1');
 pgv_select
------------
 (1,str11)
 (0,str00)

SELECT pgv_select('vars', 'r1', 1);
 pgv_select
------------
 (1,str11)

SELECT pgv_select('vars', 'r1', 0);
 pgv_select
------------
 (0,str00)

SELECT pgv_select('vars', 'r1', ARRAY[1, 0]);
 pgv_select
------------
 (1,str11)
 (0,str00)

SELECT pgv_delete('vars', 'r1', 1);

SELECT pgv_select('vars', 'r1');
 pgv_select
------------
 (0,str00)

You can list packages and variables:

SELECT * FROM pgv_list() order by package, name;
 package | name  | is_transactional
---------+-------+------------------
 vars    | arr1  | f
 vars    | int1  | f
 vars    | r1    | f
 vars    | text1 | f

And get used memory in bytes:

SELECT * FROM pgv_stats() order by package;
 package | allocated_memory
---------+------------------
 vars    |            49152

You can delete variables or whole packages:

SELECT pgv_remove('vars', 'int1');
SELECT pgv_remove('vars');

You can delete all packages and variables:

SELECT pgv_free();

If you want variables with support of transactions and savepoints, you should add flag is_transactional = true as the last argument in functions pgv_set() or pgv_insert(). Following use cases describe behavior of transactional variables:

SELECT pgv_set('pack', 'var_text', 'before transaction block'::text, true);
BEGIN;
SELECT pgv_set('pack', 'var_text', 'before savepoint'::text, true);
SAVEPOINT sp1;
SELECT pgv_set('pack', 'var_text', 'savepoint sp1'::text, true);
SAVEPOINT sp2;
SELECT pgv_set('pack', 'var_text', 'savepoint sp2'::text, true);
RELEASE sp2;
SELECT pgv_get('pack', 'var_text', NULL::text);
    pgv_get
---------------
 savepoint sp2

ROLLBACK TO sp1;
SELECT pgv_get('pack', 'var_text', NULL::text);
     pgv_get
------------------
 before savepoint

ROLLBACK;
SELECT pgv_get('pack', 'var_text', NULL::text);
         pgv_get
--------------------------
 before transaction block

If you create a transactional variable after BEGIN or SAVEPOINT statements and then rollback to previous state - variable will not be exist:

BEGIN;
SAVEPOINT sp1;
SAVEPOINT sp2;
SELECT pgv_set('pack', 'var_int', 122, true);
RELEASE SAVEPOINT sp2;
SELECT pgv_get('pack', 'var_int', NULL::int);
pgv_get
---------
     122

ROLLBACK TO sp1;
SELECT pgv_get('pack','var_int', NULL::int);
ERROR:  unrecognized variable "var_int"
COMMIT;

You can undo removal of a transactional variable by ROLLBACK, but if you remove a whole package, all regular variables will be removed permanently:

SELECT pgv_set('pack', 'var_reg', 123);
SELECT pgv_set('pack', 'var_trans', 456, true);
BEGIN;
SELECT pgv_free();
SELECT * FROM pgv_list();
 package | name | is_transactional
---------+------+------------------
(0 rows)

-- Memory is allocated yet
SELECT * FROM pgv_stats();
 package | allocated_memory
---------+------------------
 pack    |            24576

ROLLBACK;
SELECT * FROM pgv_list();
 package |   name    | is_transactional
---------+-----------+------------------
 pack    | var_trans | t

If you created transactional variable once, you should use flag is_transactional every time when you want to change variable value by functions pgv_set(), pgv_insert() and deprecated setters (i.e. pgv_set_int()). If you try to change this option, you'll get an error:

SELECT pgv_insert('pack', 'var_record', row(123::int, 'text'::text), true);

SELECT pgv_insert('pack', 'var_record', row(456::int, 'another text'::text));
ERROR:  variable "var_record" already created as TRANSACTIONAL

Functions pgv_update() and pgv_delete() do not require this flag.