PostgreSQL User's Guide
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# SET

## Name

SET — Set run-time parameters for session

    SET variable { TO | = } { value | 'value' | DEFAULT }
    SET CONSTRAINTS {ALL | constraintlist} mode
    SET TIME ZONE { 'timezone' | LOCAL | DEFAULT }
    SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }

### Inputs

`variable`

  Settable global parameter.

`value`

  New value of parameter. DEFAULT can be used to specify resetting the parameter to its default value. Lists of strings are allowed, but more complex constructs may need to be single or double quoted.

The possible variables and allowed values are:

CLIENT_ENCODING | NAMES

  Sets the multi-byte client encoding. Parameters are:

  `value`

    Sets the multi-byte client encoding to `value`. The specified encoding must be supported by the backend.

  This option is only available if MULTIBYTE support was enabled during the configure step of building Postgres.

DATESTYLE

  Set the date/time representation style. Affects the output format, and in some cases it can affect the interpretation of input.

  ISO

    use ISO 8601-style dates and times

  SQL

    use Oracle/Ingres-style dates and times

  Postgres

    use traditional Postgres format

  European

    use `dd/mm/yyyy` for numeric date representations.

  NonEuropean

    use `mm/dd/yyyy` for numeric date representations.

  German

    use `dd.mm.yyyy` for numeric date representations.

  US

    same as `NonEuropean`

  DEFAULT

    restores the default values (`ISO`)

  Date format initialization may be done by:

  Setting the `PGDATESTYLE` environment variable. If PGDATESTYLE is set in the frontend environment of a client based on libpq, libpq will automatically set DATESTYLE to the value of PGDATESTYLE during connection startup.
  Running postmaster using the option -o -e to set dates to the `European` convention. Note that this affects only some combinations of date styles; for example the ISO style is not affected by this parameter.
  Changing variables in `src/backend/utils/init/globals.c`.

  The variables in `globals.c` which can be changed are:

  bool EuroDates = false | true
  int DateStyle = USE_ISO_DATES | USE_POSTGRES_DATES | USE_SQL_DATES | USE_GERMAN_DATES

SEED

  Sets the internal seed for the random number generator.

  `value`

    The value for the seed to be used by the `random` catalog function. Significant values are floating point numbers between 0 and 1, which are then multiplied by RAND_MAX. This product will silently overflow if a number outside the range is used.

    The seed can also be set by invoking the `setseed` SQL function:

        SELECT setseed(value);

  This option is only available if MULTIBYTE support was enabled during the configure step of building Postgres.

SERVER_ENCODING

  Sets the multi-byte server encoding to:

  `value`

    The identifying value for the server encoding.

  This option is only available if MULTIBYTE support was enabled during the configure step of building Postgres.

CONSTRAINTS

  SET CONSTRAINTS affects the behavior of constraint evaluation in the current transaction. SET CONSTRAINTS, specified in SQL3, has these allowed parameters:

  `constraintlist`

    Comma separated list of deferrable constraint names.

  `mode`

    The constraint mode. Allowed values are DEFERRED and IMMEDIATE.

  In IMMEDIATE mode, foreign key constraints are checked at the end of each query.

  In DEFERRED mode, foreign key constraints marked as DEFERRABLE are checked only at transaction commit or until its mode is explicitly set to IMMEDIATE. This is actually only done for foreign key constraints, so it does not apply to UNIQUE or other constraints.

TIME ZONE, TIMEZONE

  The possible values for timezone depends on your operating system. For example on Linux /usr/lib/zoneinfo contains the database of timezones.

  Here are some valid values for timezone:

  PST8PDT

    set the timezone for California

  Portugal

    set time zone for Portugal.

  'Europe/Rome'

    set time zone for Italy.

  DEFAULT

    set time zone to your local timezone (value of the TZ environment variable).

  If an invalid time zone is specified, the time zone becomes GMT (on most systems anyway).

  The second syntax shown above, allows one to set the timezone with a syntax similar to SQL92 SET TIME ZONE. The LOCAL keyword is just an alternate form of DEFAULT for SQL92 compatibility.

  If the PGTZ environment variable is set in the frontend environment of a client based on libpq, libpq will automatically set TIMEZONE to the value of PGTZ during connection startup.

TRANSACTION ISOLATION LEVEL

  Sets the isolation level for the current transaction.

  READ COMMITTED

    The current transaction queries read only rows committed before a query began. READ COMMITTED is the default.

    > Note: SQL92 standard requires SERIALIZABLE to be the default isolation level.

  SERIALIZABLE

    The current transaction queries read only rows committed before first DML statement (SELECT/INSERT/DELETE/UPDATE/FETCH/COPY_TO) was executed in this transaction.

There are also several internal or optimization parameters which can be specified by the SET command:

PG_OPTIONS

  Sets various backend parameters.

RANDOM_PAGE_COST

  Sets the optimizer's estimate of the cost of a nonsequentially fetched disk page. This is measured as a multiple of the cost of a sequential page fetch.

  `float8`

    Set the cost of a random page access to the specified floating-point value.

CPU_TUPLE_COST

  Sets the optimizer's estimate of the cost of processing each tuple during a query. This is measured as a fraction of the cost of a sequential page fetch.

  `float8`

    Set the cost of per-tuple CPU processing to the specified floating-point value.

CPU_INDEX_TUPLE_COST

  Sets the optimizer's estimate of the cost of processing each index tuple during an index scan. This is measured as a fraction of the cost of a sequential page fetch.

  `float8`

    Set the cost of per-index-tuple CPU processing to the specified floating-point value.

CPU_OPERATOR_COST

  Sets the optimizer's estimate of the cost of processing each operator in a WHERE clause. This is measured as a fraction of the cost of a sequential page fetch.

  `float8`

    Set the cost of per-operator CPU processing to the specified floating-point value.

EFFECTIVE_CACHE_SIZE

  Sets the optimizer's assumption about the effective size of the disk cache (that is, the portion of the kernel's disk cache that will be used for Postgres data files). This is measured in disk pages, which are normally 8Kb apiece.

  `float8`

    Set the assumed cache size to the specified floating-point value.

ENABLE_SEQSCAN

  Enables or disables the planner's use of sequential scan plan types. (It's not possible to suppress sequential scans entirely, but turning this variable OFF discourages the planner from using one if there is any other method available.)

  ON

    enables use of sequential scans (default setting).

  OFF

    disables use of sequential scans.

ENABLE_INDEXSCAN

  Enables or disables the planner's use of index scan plan types.

  ON

    enables use of index scans (default setting).

  OFF

    disables use of index scans.

ENABLE_TIDSCAN

  Enables or disables the planner's use of TID scan plan types.

  ON

    enables use of TID scans (default setting).

  OFF

    disables use of TID scans.

ENABLE_SORT

  Enables or disables the planner's use of explicit sort steps. (It's not possible to suppress explicit sorts entirely, but turning this variable OFF discourages the planner from using one if there is any other method available.)

  ON

    enables use of sorts (default setting).

  OFF

    disables use of sorts.

ENABLE_NESTLOOP

  Enables or disables the planner's use of nested-loop join plans. (It's not possible to suppress nested-loop joins entirely, but turning this variable OFF discourages the planner from using one if there is any other method available.)

  ON

    enables use of nested-loop joins (default setting).

  OFF

    disables use of nested-loop joins.

ENABLE_MERGEJOIN

  Enables or disables the planner's use of mergejoin plans.

  ON

    enables use of merge joins (default setting).

  OFF

    disables use of merge joins.

ENABLE_HASHJOIN

  Enables or disables the planner's use of hashjoin plans.

  ON

    enables use of hash joins (default setting).

  OFF

    disables use of hash joins.

GEQO

  Sets the threshold for using the genetic optimizer algorithm.

  ON

    enables the genetic optimizer algorithm for statements with 11 or more tables. (This is also the DEFAULT setting.)

  ON=`#`

    Takes an integer argument to enable the genetic optimizer algorithm for statements with `#` or more tables in the query.

  OFF

    disables the genetic optimizer algorithm.

  See the chapter on GEQO in the Programmer's Guide for more information about query optimization.

  If the PGGEQO environment variable is set in the frontend environment of a client based on libpq, libpq will automatically set GEQO to the value of PGGEQO during connection startup.

KSQO

  Key Set Query Optimizer causes the query planner to convert queries whose WHERE clause contains many OR'ed AND clauses (such as "WHERE (a=1 AND b=2) OR (a=2 AND b=3) ...") into a UNION query. This method can be faster than the default implementation, but it doesn't necessarily give exactly the same results, since UNION implicitly adds a SELECT DISTINCT clause to eliminate identical output rows. KSQO is commonly used when working with products like MicroSoft Access, which tend to generate queries of this form.

  ON

    enables this optimization.

  OFF

    disables this optimization (default setting).

  DEFAULT

    Equivalent to specifying SET KSQO=OFF.

  The KSQO algorithm used to be absolutely essential for queries with many OR'ed AND clauses, but in Postgres 7.0 and later the standard planner handles these queries fairly successfully.

MAX_EXPR_DEPTH

  Sets the maximum expression nesting depth that the parser will accept. The default value is high enough for any normal query, but you can raise it if you need to. (But if you raise it too high, you run the risk of backend crashes due to stack overflow.)

  `integer`

    Maximum depth.

### Outputs

`SET VARIABLE`

  Message returned if successful.

`WARN: Bad value for `variable` (`value`)`

  If the command fails to set the specified variable.

## Description

SET will modify configuration parameters for variable during a session.

Current values can be obtained using SHOW, and values can be restored to the defaults using RESET. Parameters and values are case-insensitive. Note that the value field is always specified as a string, so is enclosed in single-quotes.

SET TIME ZONE changes the session's default time zone offset. An SQL-session always begins with an initial default time zone offset. The SET TIME ZONE statement is used to change the default time zone offset for the current SQL session.

### Notes

The SET `variable` statement is a Postgres language extension.

Refer to SHOW and RESET to display or reset the current values.

## Usage

Set the style of date to ISO (no quotes on the argument is required):

    SET DATESTYLE TO ISO;

Enable GEQO for queries with 4 or more tables (note the use of single quotes to handle the equal sign inside the value argument):

    SET GEQO = 'ON=4';

Set GEQO to default:

    SET GEQO = DEFAULT;

Set the timezone for Berkeley, California, using double quotes to preserve the uppercase attributes of the time zone specifier:

    SET TIME ZONE "PST8PDT";
    SELECT CURRENT_TIMESTAMP AS today;

             today
    ------------------------
     1998-03-31 07:41:21-08

Set the timezone for Italy (note the required single or double quotes to handle the special characters):

    SET TIME ZONE 'Europe/Rome';
    SELECT CURRENT_TIMESTAMP AS today;

             today
    ------------------------
     1998-03-31 17:41:31+02

## Compatibility

### SQL92

There is no general SET `variable` in SQL92 (with the exception of SET TRANSACTION ISOLATION LEVEL). The SQL92 syntax for SET TIME ZONE is slightly different, allowing only a single integer value for time zone specification:

    SET TIME ZONE { interval_value_expression | LOCAL }

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