pg_variables - session variables with various types

Introduction

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.

License

This module available under the license similar to PostgreSQL.

Installation

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;"

Module functions

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

Scalar variables functions

Function	Returns
pgv_set(package text, name text, value anynonarray, is_transactional bool default false)	void
pgv_get(package text, name text, var_type anynonarray, strict bool default true)	anynonarray

Array variables functions

Function	Returns
pgv_set(package text, name text, value anyarray, is_transactional bool default false)	void
pgv_get(package text, name text, var_type anyarray, strict bool default true)	anyarray

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.

Deprecated scalar variables functions

Integer variables

Function	Returns
pgv_set_int(package text, name text, value int, is_transactional bool default false)	void
pgv_get_int(package text, name text, strict bool default true)	int

Text variables

Function	Returns
pgv_set_text(package text, name text, value text, is_transactional bool default false)	void
pgv_get_text(package text, name text, strict bool default true)	text

Numeric variables

Function	Returns
pgv_set_numeric(package text, name text, value numeric, is_transactional bool default false)	void
pgv_get_numeric(package text, name text, strict bool default true)	numeric

Timestamp variables

Function	Returns
pgv_set_timestamp(package text, name text, value timestamp, is_transactional bool default false)	void
pgv_get_timestamp(package text, name text, strict bool default true)	timestamp

Timestamp with timezone variables

Function	Returns
pgv_set_timestamptz(package text, name text, value timestamptz, is_transactional bool default false)	void
pgv_get_timestamptz(package text, name text, strict bool default true)	timestamptz

Date variables

Function	Returns
pgv_set_date(package text, name text, value date, is_transactional bool default false)	void
pgv_get_date(package text, name text, strict bool default true)	date

Jsonb variables

Function	Returns
pgv_set_jsonb(package text, name text, value jsonb, is_transactional bool default false)	void
pgv_get_jsonb(package text, name text, strict bool default true)	jsonb

Record variables functions

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.

Function	Returns	Description
pgv_insert(package text, name text, r record, is_transactional bool default false)	void	Inserts a record to the variable collection. If package and variable do not exists they will be created. The first column of r will be a primary key. If exists a record with the same primary key the error will be raised. If this variable collection has other structure the error will be raised.
pgv_update(package text, name text, r record)	boolean	Updates a record with the corresponding primary key (the first column of r is a primary key). Returns true if a record was found. If this variable collection has other structure the error will be raised.
pgv_delete(package text, name text, value anynonarray)	boolean	Deletes a record with the corresponding primary key (the first column of r is a primary key). Returns true if a record was found.
pgv_select(package text, name text)	set of record	Returns the variable collection records.
pgv_select(package text, name text, value anynonarray)	record	Returns the record with the corresponding primary key (the first column of r is a primary key).
pgv_select(package text, name text, value anyarray)	set of record	Returns the variable collection records with the corresponding primary keys (the first column of r is a primary key).

Miscellaneous functions

Function	Returns	Description
pgv_exists(package text, name text)	bool	Returns true if package and variable exists.
pgv_exists(package text)	bool	Returns true if package exists.
pgv_remove(package text, name text)	void	Removes the variable with the corresponding name. Required package and variable must exists, otherwise the error will be raised.
pgv_remove(package text)	void	Removes the package and all package variables with the corresponding name. Required package must exists, otherwise the error will be raised.
pgv_free()	void	Removes all packages and variables.
pgv_list()	table(package text, name text, is_transactional bool)	Returns set of records of assigned packages and variables.
pgv_stats()	table(package text, allocated_memory bigint)	Returns list of assigned packages and used memory in bytes.

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

Examples

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.