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 SQL SecuritySupplementary Information 

Management statement are a class of SQL statements for administering aspects of the client/server environment, usually for the current session.Typically, such statements start with the verb SET.

Note

The isql tool also has a collection of SET commands.Those commands are not part of Firebird’s SQL lexicon.For information on isqls SET commands, seeIsql Set Commandsin Firebird Interactive SQL Utility.

Management statements can run anywhere DSQL can run, but typically, the developer will want to run a management statement in a database trigger.A subset of management statement can be used directly in PSQL modules without the need to wrap them in an EXECUTE STATEMENT block.For more details of the current set, see Management Statements in PSQL in the PSQL chapter.

Most of the management statements affect the current connection (attachment, or “session”) only, and do not require any authorization over and above the login privileges of the current user without elevated privileges.

Some management statements operate beyond the scope of the current session.Examples are the ALTER DATABASE {BEGIN | END} BACKUP statements to control the “copy-safe” mode, or the ALTER EXTERNAL CONNECTIONS POOL statements to manage connection pooling.A set of system privileges, analogous with SQL privileges granted for database objects, is provided to enable the required authority to run a specific management statement in this category.

Note

Some statements of this class use the verb ALTER, although management statements should not be confused with DDL ALTER statements that modify database objects like tables, views, procedures, roles, et al.

Although some ALTER DATABASE clauses (e.g. BEGIN BACKUP) can be considered as management statements, they are documented in the DDL chapter.

Unless explicitly mentioned otherwise in an “Available in” section, management statements are available in DSQL and PSQL.Availability in ESQL is not tracked by this Language Reference.

Data Type Behaviour

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SET BIND (Data Type Coercion Rules)

Configures data type coercion rules for the current session

Syntax
SET BIND OF <type_from> TO <type_to>

<type_from> ::=
    <scalar_datatype>
  | <blob_datatype>
  | TIME ZONE
  | VARCHAR | {CHARACTER | CHAR} VARYING

<type_to> ::=
    <scalar_datatype>
  | <blob_datatype>
  | VARCHAR | {CHARACTER | CHAR} VARYING
  | LEGACY | NATIVE | EXTENDED
  | EXTENDED TIME WITH TIME ZONE
  | EXTENDED TIMESTAMP WITH TIME ZONE

<scalar_datatype> ::=
  !! See Scalar Data Types Syntax !!

<blob_datatype> ::=
  !! See BLOB Data Types Syntax !!

This statement makes it possible to substitute one data type with another when performing client-server interactions.In other words, type_from returned by the engine is represented as type_to in the client API.

Note

Only fields returned by the database engine in regular messages are substituted according to these rules.Variables returned as an array slice are not affected by the SET BIND statement.

When a partial type definition is used (e.g. CHAR instead of CHAR(n)) in from_type, the coercion is performed for all CHAR columns.The special partial type TIME ZONE stands for TIME WITH TIME ZONE and TIMESTAMP WITH TIME ZONE.When a partial type definition is used in to_type, the engine defines missing details about that type automatically based on source column.

Changing the binding of any NUMERIC or DECIMAL data type does not affect the underlying integer type.In contrast, changing the binding of an integer data type also affects appropriate NUMERIC and DECIMAL types.For example, SET BIND OF INT128 TO DOUBLE PRECISION will also map NUMERIC and DECIMAL with precision 19 or higher, as these types use INT128 as their underlying type.

The special type LEGACY is used when a data type, missing in previous Firebird version, should be represented in a way, understandable by old client software (possibly with data loss).The coercion rules applied in this case are shown in the table below.

Table 1. Native to LEGACY coercion rules
Native data type Legacy data type

BOOLEAN

CHAR(5)

DECFLOAT

DOUBLE PRECISION

INT128

BIGINT

TIME WITH TIME ZONE

TIME WITHOUT TIME ZONE

TIMESTAMP WITH TIME ZONE

TIMESTAMP WITHOUT TIME ZONE

Using EXTENDED for type_to causes the engine to coerce to an extended form of the type_from data type.Currently, this works only for TIME/TIMESTAMP WITH TIME ZONE, they are coerced to EXTENDED TIME/TIMESTAMP WITH TIME ZONE.The EXTENDED type contains both the time zone name, and the corresponding GMT offset, so it remains usable if the client application cannot process named time zones properly (e.g. due to the missing ICU library).

Setting a binding to NATIVE resets the existing coercion rule for this data type and returns it in its native format.

The initial bind rules of a connection be configured through the DPB by providing a semicolon separated list of <type_from> TO <type_to> options as the string value of isc_dpb_set_bind.

Execution of [fblangref50-management-session-reset-alter] will revert to the binding rules configured through the DPB, or otherwise the system default.

Tip

It is also possible to configure a default set of data type coercion rules for all clients through the DataTypeCompatibility configuration option, either as a global configuration in firebird.conf or per database in databases.conf.

DataTypeCompatibility currently has two possible values: 3.0 and 2.5.The 3.0 option maps data types introduced after Firebird 3.0 — in particular DECIMAL/NUMERIC with precision 19 or higher, INT128, DECFLOAT, and TIME/TIMESTAMP WITH TIME ZONE — to data types supported in Firebird 3.0.The 2.5 option also converts the BOOLEAN data type.

See the [fblangref50-management-legacy-coercion-rules] for details.This setting allows legacy client applications to work with Firebird 5.0 without recompiling or otherwise adjusting them to understand the new data types.

SET BIND Examples

-- native
SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE;

                   CAST
=======================
                 123.45

-- double
SET BIND OF DECFLOAT TO DOUBLE PRECISION;
SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE;

                   CAST
=======================
      123.4500000000000

-- still double
SET BIND OF DECFLOAT(34) TO CHAR;
SELECT CAST('123.45' AS DECFLOAT(16)) FROM RDB$DATABASE;

                   CAST
=======================
      123.4500000000000

-- text
SELECT CAST('123.45' AS DECFLOAT(34)) FROM RDB$DATABASE;

CAST
==========================================
123.45

In the case of missing ICU on the client side:

SELECT CURRENT_TIMESTAMP FROM RDB$DATABASE;

                                        CURRENT_TIMESTAMP
=========================================================
2020-02-21 16:26:48.0230 GMT*

SET BIND OF TIME ZONE TO EXTENDED;
SELECT CURRENT_TIMESTAMP FROM RDB$DATABASE;

                                        CURRENT_TIMESTAMP
=========================================================
2020-02-21 19:26:55.6820 +03:00

SET DECFLOAT

Configures DECFLOAT rounding and error behaviour for the current session

Syntax
SET DECFLOAT
  { ROUND <round_mode>
  | TRAPS TO [<trap_opt> [, <trap_opt> ...]] }

<round_mode> ::=
    CEILING | UP | HALF_UP | HALF_EVEN
  | HALF_DOWN | DOWN | FLOOR | REROUND

<trap_opt> ::=
    DIVISON_BY_ZERO | INEXACT | INVALID_OPERATION
  | OVERFLOW | UNDERFLOW

SET DECFLOAT ROUND

SET DECFLOAT ROUND changes the rounding behaviour of operations on DECFLOAT.The default rounding mode is HALF_UP.The initial configuration of a connection can also be specified using the DPB tag isc_dpb_decfloat_round with the desired round_mode as string value.

The valid rounding modes are:

CEILING

towards +infinity
UP

away from 0
HALF_UP

to nearest, if equidistant, then up (default)
HALF_EVEN

to nearest, if equidistant, ensure last digit in the result will be even
HALF_DOWN

to nearest, if equidistant, then down
DOWN

towards 0
FLOOR

towards -infinity
REROUND

up if digit to be rounded is 0 or 5, down in other cases

The current value for the connection can be found using RDB$GET_CONTEXT('SYSTEM', 'DECFLOAT_ROUND').

Execution of [fblangref50-management-session-reset-alter] will revert to the value configured through the DPB, or otherwise the system default.

SET DECFLOAT TRAPS

SET DECFLOAT TRAPS changes the error behaviour of operations on DECFLOAT.The default traps are DIVISION_BY_ZERO,INVALID_OPERATION,OVERFLOW;this default matches the behaviour specified in the SQL standard for DECFLOAT.This statement controls whether certain exceptional conditions result in an error (“trap”) or alternative handling (for example, an underflow returns 0 when not set, or an overflow returns an infinity).The initial configuration of a connection can also be specified using the DPB tag isc_dpb_decfloat_traps with the desired comma-separated trap_opt values as a string value.

Valid trap options (exceptional conditions) are:

Division_by_zero

(set by default)
Inexact

 — 
Invalid_operation

(set by default)
Overflow

(set by default)
Underflow

 — 

The current value for the connection can be found using RDB$GET_CONTEXT('SYSTEM', 'DECFLOAT_TRAPS').

Execution of [fblangref50-management-session-reset-alter] will revert to the value configured through the DPB, or otherwise the system default.

Connections Pool Management

Management statements to manage the external connections pool.

This connection pool is part of the Firebird server and used for connections to other databases or servers from the Firebird server itself.

ALTER EXTERNAL CONNECTIONS POOL

Manages the external connections pool

Syntax
ALTER EXTERNAL CONNECTIONS POOL
  { CLEAR ALL
  | CLEAR OLDEST
  | SET LIFETIME lifetime <time-unit>
  | SET SIZE size }

<time-unit> ::= SECOND | MINUTE | HOUR
Table 1. ALTER EXTERNAL CONNECTIONS POOL Statement Parameters
Parameter Description

lifetime

Maximum lifetime of a connection in the pool.Minimum values is 1 SECOND, maximum is 24 HOUR.

size

Maximum size of the connection pool.Range 0 - 1000.Setting to 0 disables the external connections pool.

When prepared it is described like a DDL statement, but its effect is immediate — it is executed immediately and to completion, without waiting for transaction commit.

This statement can be issued from any connection, and changes are applied to the in-memory instance of the pool in the current Firebird process.If the process is Firebird Classic, execution only affects the current process (current connection), and does not affect other Classic processes.

Changes made with ALTER EXTERNAL CONNECTIONS POOL are not persistent: after a restart, Firebird will use the pool settings configured in firebird.conf by ExtConnPoolSize and ExtConnPoolLifeTime.

Clauses of ALTER EXTERNAL CONNECTIONS POOL

CLEAR ALL

Closes all idle connections and disassociates currently active connections;they are immediately closed when unused.

CLEAR OLDEST

Closes expired connections

SET LIFETIME

Configures the maximum lifetime of an idle connection in the pool.The default value (in seconds) is set using the parameter ExtConnPoolLifetime in firebird.conf.

SET SIZE

Configures the maximum number of idle connections in the pool.The default value is set using the parameter ExtConnPoolSize in firebird.conf.

How the Connection Pool Works

Every successful connection is associated with a pool, which maintains two lists — one for idle connections and one for active connections.When a connection in the “active” list has no active requests and no active transactions, it is assumed to be “unused”.A reset of the unused connection is attempted using an [fblangref50-management-session-reset-alter] statement and,

  • if the reset succeeds (no errors occur) the connection is moved into the “idle” list;

  • if the reset fails, the connection is closed;

  • if the pool has reached its maximum size, the oldest idle connection is closed.

  • When the lifetime of an idle connection expires, it is deleted from the pool and closed.

New Connections

When the engine is asked to create a new external connection, the pool first looks for a candidate in the “idle” list.The search, which is case-sensitive, involves four parameters:

  1. connection string

  2. username

  3. password

  4. role

If a suitable connection is found, it is tested to check that it is still alive.

  • If it fails the check, it is deleted, and the search is repeated, without reporting any error to the client

  • Otherwise, the live connection is moved from the “idle” list to the “active” list and returned to the caller

  • If there are multiple suitable connections, the most recently used one is chosen

  • If there is no suitable connection, a new one is created and added to the “active” list.

Who Can Alter the External Connections Pool

The ALTER EXTERNAL CONNECTIONS POOL statement can be executed by:

See also

RDB$GET_CONTEXT

Changing the Current Role

SET ROLE

Sets the active role of the current session

Available in

DSQL

Syntax
SET ROLE {role_name | NONE}
Table 1. SET ROLE Statement Parameters
Parameter Description

role_name

The name of the role to apply

The SET ROLE statement allows a user to assume a different role;it sets the CURRENT_ROLE context variable to role_name, if that role has been granted to the CURRENT_USER.For this session, the user receives the privileges granted by that role.Any rights granted to the previous role are removed from the session.Use NONE instead of role_name to clear the CURRENT_ROLE.

When the specified role does not exist or has not been explicitly granted to the user, the error “Role role_name is invalid or unavailable” is raised.

SET ROLE Examples

  1. Change the current role to MANAGER

    SET ROLE manager;
select current_role from rdb$database;

ROLE
=======================
MANAGER

  2. Clear the current role

    SET ROLE NONE;
select current_role from rdb$database;

ROLE
=======================
NONE
See also

[fblangref50-management-role-set-trusted], GRANT

SET TRUSTED ROLE

Sets the active role of the current session to the trusted role

Available in

DSQL

Syntax
SET TRUSTED ROLE

The SET TRUSTED ROLE statement makes it possible to assume the role assigned to the user through a mapping rule (see Mapping of Users to Objects).The role assigned through a mapping rule is assumed automatically on connect, if the user hasn’t specified an explicit role.The SET TRUSTED ROLE statement makes it possible to assume the mapped (or “trusted”) role at a later time, or to assume it again after the current role was changed using SET ROLE.

A trusted role is not a specific type of role, but can be any role that was created using CREATE ROLE, or a predefined system role such as RDB$ADMIN.An attachment (session) has a trusted role when the security objects mapping subsystem finds a match between the authentication result passed from the plugin and a local or global mapping to a role for the current database.The role may be one that is not granted explicitly to that user.

When a session has no trusted role, executing SET TRUSTED ROLE will raise error “Your attachment has no trusted role”.

Note

While the CURRENT_ROLE can be changed using SET ROLE, it is not always possible to revert to a trusted role using the same command, because SET ROLE checks if the role has been granted to the user.With SET TRUSTED ROLE, the trusted role can be assumed again even when SET ROLE fails.

SET TRUSTED ROLE Examples

  1. Assuming a mapping rule that assigns the role ROLE1 to a user ALEX:

    CONNECT 'employee' USER ALEX PASSWORD 'password';
SELECT CURRENT_ROLE FROM RDB$DATABASE;

ROLE
===============================
ROLE1

SET ROLE ROLE2;
SELECT CURRENT_ROLE FROM RDB$DATABASE;

ROLE
===============================
ROLE2

SET TRUSTED ROLE;
SELECT CURRENT_ROLE FROM RDB$DATABASE;

ROLE
===============================
ROLE1
See also

[fblangref50-management-role-set], Mapping of Users to Objects

Session Timeouts

Statements for management of timeouts of the current connection.

SET SESSION IDLE TIMEOUT

Sets the session idle timeout

Syntax
SET SESSION IDLE TIMEOUT value [<time-unit>]

<time-unit> ::= MINUTE | HOUR | SECOND
Table 1. SET SESSION IDLE TIMEOUT Statement Parameters
Parameter Description

value

The timeout duration expressed in time-unit.A value of 0 defers to connection idle timeout configured for the database.

time-unit

Time unit of the timeout.Defaults to MINUTE.

The SET SESSION IDLE TIMEOUT sets an idle timeout at connection level and takes effect immediately.The statement can run outside transaction control (without an active transaction).

Setting a value larger than configured for the database is allowed, but is effectively ignored, see also [fblangref50-management-session-timeout-effective].

The current timeout set for the session can be retrieved through RDB$GET_CONTEXT, namespace SYSTEM and variable SESSION_IDLE_TIMEOUT.Information is also available from MON$ATTACHMENTS:

MON$IDLE_TIMEOUT

Connection-level idle timeout in seconds;0 if timeout is not set.

MON$IDLE_TIMER

Idle timer expiration time;contains NULL if an idle timeout was not set, or if a timer is not running.

Both RDB$GET_CONTEXT('SYSTEM', 'SESSION_IDLE_TIMEOUT') and MON$ATTACHMENTS.MON$IDLE_TIMEOUT report the idle timeout configured for the connection;they do not report the effective idle timeout.

The session idle timeout is reset when [fblangref50-management-session-reset-alter] is executed.

Idle Session Timeouts

An idle session timeout allows a use connection to close automatically after a specified period of inactivity.A database administrator can use it to enforce closure of old connections that have become inactive, to reduce unnecessary consumption of resources.It can also be used by application and tools developers as an alternative to writing their own modules for controlling connection lifetime.

By default, the idle timeout is not enabled.No minimum or maximum limit is imposed, but a reasonably large period — such as a few hours — is recommended.

How the Idle Session Timeout Works

  • When the user API call leaves the engine (returns to the calling connection) a special idle timer associated with the current connection is started

  • When another user API call from that connection enters the engine, the idle timer is stopped and reset to zero

  • If the maximum idle time is exceeded, the engine immediately closes the connection in the same way as with asynchronous connection cancellation:

    • all active statements and cursors are closed

    • all active transactions are rolled back

    • The network connection remains open at this point, allowing the client application to get the exact error code on the next API call.The network connection will be closed on the server side, after an error is reported or in due course as a result of a network timeout from a client-side disconnection.

Note

Whenever a connection is cancelled, the next user API call returns the error isc_att_shutdown with a secondary error specifying the exact reason.Now, we have

isc_att_shut_idle

Idle timeout expired

in addition to

isc_att_shut_killed

Killed by database administrator

isc_att_shut_db_down

Database is shut down

isc_att_shut_engine

Engine is shut down
Setting the Idle Session Timeout
Note

The idle timer will not start if the timeout period is set to zero.

An idle session timeout can be set:

  • At database level, the database administrator can set the configuration parameter ConnectionIdleTimeout, an integer value in minutes.The default value of zero means no timeout is set.It is configurable per-database, so it may be set globally in firebird.conf and overridden for individual databases in databases.conf as required.

    The scope of this method is all user connections, except system connections (garbage collector, cache writer, etc.).

  • at connection level, the idle session timeout is supported by both the SET SESSION IDLE TIMEOUT statement and the API (setIdleTimeout).The scope of this method is specific to the supplied connection (attachment).Its value in the API is in seconds.In the SQL syntax it can be hours, minutes or seconds.Scope for this method is the connection to which it is applied.

Note

For more information about the API calls, consult theFirebird 4.0 Release Notes.
Determining the Timeout that is In Effect

The effective idle timeout value is determined whenever a user API call leaves the engine, checking first at connection level and then at database level.A connection-level timeout can override the value of a database-level setting, as long as the period of time for the connection-level setting is no longer than any non-zero timeout that is applicable at database level.

Important

Take note of the difference between the time units at each level.At database level, in the configuration files, the unit for SessionTimeout is minutes.In SQL, the default unit is minutes but can also be expressed in hours or seconds explicitly.At the API level, the unit is seconds.

Absolute precision is not guaranteed in any case, especially when the system load is high, but timeouts are guaranteed not to expire earlier than the moment specified.

SET STATEMENT TIMEOUT

Sets the statement timeout for a connection

Syntax
SET STATEMENT TIMEOUT value [<time-unit>]

<time-unit> ::= SECOND | MILLISECOND | MINUTE | HOUR
Table 1. SET STATEMENT TIMEOUT Statement Parameters
Parameter Description

value

The timeout duration expressed in time-unit.A value of 0 defers to statement timeout configured for the database.

time-unit

Time unit of the timeout.Defaults to SECOND.

The SET STATEMENT TIMEOUT sets a statement timeout at connection level and takes effect immediately.The statement can run outside transaction control (without an active transaction).

Setting a value larger than configured for the database is allowed, but is effectively ignored, see also [fblangref50-management-stmnt-timeout-effective].

The current statement timeout set for the session can be retrieved through RDB$GET_CONTEXT, namespace SYSTEM and variable STATEMENT_TIMEOUT.Information is also available from MON$ATTACHMENTS:

MON$STATEMENT_TIMEOUT

Connection-level statement timeout in milliseconds;0 if timeout is not set.

In MON$STATEMENTS:

MON$STATEMENT_TIMEOUT

Statement-level statement timeout in milliseconds;0 if timeout is not set.

MON$STATEMENT_TIMER

Timeout timer expiration time;contains NULL if an idle timeout was not set, or if a timer is not running.

Both RDB$GET_CONTEXT('SYSTEM', 'STATEMENT_TIMEOUT') and MON$ATTACHMENTS.MON$STATEMENT_TIMEOUT report the statement timeout configured for the connection, and MON$STATEMENTS.MON$STATEMENT_TIMEOUT for the statement;they do not report the effective statement timeout.

The statement timeout is reset when [fblangref50-management-session-reset-alter] is executed.

Statement Timeouts

The statement timeout feature allows execution of a statement to be stopped automatically when it has been running longer than a given timeout period.It gives the database administrator an instrument for limiting excessive resource consumption from heavy queries.

Statement timeouts can also be useful to application developers when creating and debugging complex queries without advance knowledge of execution time.Testers and others could find them handy for detecting long-running queries and establishing finite run times for test suites.

How the Statement Timeout Works

When the statement starts execution, or a cursor is opened, the engine starts a special timer.It is stopped when the statement completes execution, or the last record has been fetched by the cursor.

Note

A fetch does not reset this timer.

When the timeout point is reached:

  • if statement execution is active, it stops at closest possible moment

  • if statement is not active currently (between fetches, for example), it is marked as cancelled, and the next fetch will break execution and return an error

Note
Statement types excluded from timeouts

Statement timeouts are not applicable to some types of statement and will be ignored:

  • All DDL statements

  • All internal queries issued by the engine itself
Setting a Statement Timeout
Note

The timer will not start if the timeout period is set to zero.

A statement timeout can be set:

  • at database level, by the database administrator, by setting the configuration parameter StatementTimeout in firebird.conf or databases.conf.StatementTimeout is an integer representing the number of seconds after which statement execution will be cancelled automatically by the engine.Zero means no timeout is set.A non-zero setting will affect all statements in all connections.

  • at connection level, using SET STATEMENT TIMEOUT or the API for setting a statement timeout (setStatementTimeout).A connection-level setting (via SQL or the API) affects all statements for the given connection;units for the timeout period at this level can be specified to any granularity from hours to milliseconds.

  • at statement level, using the API, in milliseconds

Determining the Statement Timeout that is In Effect

The statement timeout value that is in effect is determined whenever a statement starts executing, or a cursor is opened.In searching out the timeout in effect, the engine goes up through the levels, from statement through to database and/or global levels until it finds a non-zero value.If the value in effect turns out to be zero then no statement timer is running and no timeout applies.

A statement-level or connection-level timeout can override the value of a database-level setting, as long as the period of time for the lower-level setting is no longer than any non-zero timeout that is applicable at database level.

Important

Take note of the difference between the time units at each level.At database level, in the conf file, the unit for StatementTimeout is seconds.In SQL, the default unit is seconds but can be expressed in hours, minutes or milliseconds explicitly.At the API level, the unit is milliseconds.

Absolute precision is not guaranteed in any case, especially when the system load is high, but timeouts are guaranteed not to expire earlier than the moment specified.

Whenever a statement times out and is cancelled, the next user API call returns the error isc_cancelled with a secondary error specifying the exact reason, viz.,

isc_cfg_stmt_timeout

Config level timeout expired

isc_att_stmt_timeout

Attachment level timeout expired

isc_req_stmt_timeout

Statement level timeout expired

Note
Notes about Statement Timeouts

  1. A client application could wait longer than the time set by the timeout value if the engine needs to undo a large number of actions as a result of the statement cancellation

  2. When the engine runs an EXECUTE STATEMENT statement, it passes the remainder of the currently active timeout to the new statement.If the external (remote) engine does not support statement timeouts, the local engine silently ignores any corresponding error.

  3. When the engine acquires a lock from the lock manager, it tries to lower the value of the lock timeout using the remainder of the currently active statement timeout, if possible.Due to lock manager internals, any statement timeout remainder will be rounded up to whole seconds.

Time Zone Management

Statements for management of time zone features of the current connections.

SET TIME ZONE

Sets the session time zone

Syntax
SET TIME ZONE { time_zone_string | LOCAL }

Changes the session time zone to the specified time zone.Specifying LOCAL will revert to initial session time zone of the session (either the default or as specified through connection property isc_dpb_session_time_zone).

Executing [fblangref50-management-session-reset-alter] has the same effect on the session time zone as SET TIME ZONE LOCAL, but will also reset other session properties.

SET TIME ZONE Examples

set time zone '-02:00';
set time zone 'America/Sao_Paulo';
set time zone local;

Optimizer Configuration

SET OPTIMIZE

Configures whether the optimizer should optimize for fetching first or all rows.

Syntax
SET OPTIMIZE <optimize-mode>

<optimize-mode> ::=
    FOR {FIRST | ALL} ROWS
  | TO DEFAULT

This feature allows the optimizer to consider another (hopefully better) plan if only a subset or rows is fetched initially by the user application (with the remaining rows being fetched on demand), thus improving the response time.

It can also be specified at the statement level using the OPTIMIZE FOR clause.

The default behaviour can be specified globally using the OptimizeForFirstRows setting in firebird.conf or databases.conf.

Reset Session State

ALTER SESSION RESET

Resets the session state of the current connection to its initial values

Syntax
ALTER SESSION RESET

Resetting the session can be useful for reusing the connection by a client application (for example, by a client-side connection pool).When this statement is executed, all user context variables are cleared, contents of global temporary tables are cleared, and all session-level settings are reset to their initial values.

It is possible to execute ALTER SESSION RESET without a transaction.

Execution of ALTER SESSION RESET performs the following steps:

  • Error isc_ses_reset_err (335545206) is raised if any transaction is active in the current session other than the current transaction (the one executing ALTER SESSION RESET) and two-phase transactions in the prepared state.

  • System variable RESETTING is set to TRUE.

  • ON DISCONNECT database triggers are fired, if present and if database triggers are not disabled for the current connection.

  • The current transaction (the one executing ALTER SESSION RESET), if present, is rolled back.A warning is reported if this transaction modified data before resetting the session.

  • Session configuration is reset to their initial values.This includes, but is not limited to:

    • DECFLOAT parameters (TRAP and ROUND) are reset to the initial values defined using the DPB at connect time, or otherwise the system default.

    • Session and statement timeouts are reset to zero.

    • The current role is restored to the initial value defined using DPB at connect time, and — if the role changed — the security classes cache is cleared.

    • The session time zone is reset to the initial value defined using the DPB at connect time, or otherwise the system default.

    • The bind configuration is reset to the initial value defined using the DPB at connect time, or otherwise the database or system default.

    • In general, configuration values should revert to the values configured using the DPB at connect time, or otherwise the database or system default.

  • Context variables defined for the USER_SESSION namespace are removed (USER_TRANSACTION was cleared earlier by the transaction roll back).

  • Global temporary tables defined as ON COMMIT PRESERVE ROWS are truncated (their contents is cleared).

  • ON CONNECT database triggers are fired, if present and if database triggers are not disabled for the current connection.

  • A new transaction is implicitly started with the same parameters as the transaction that was rolled back (if there was a transaction)

  • System variable RESETTING is set to FALSE.

Note

  • The context variables CURRENT_USER and CURRENT_CONNECTION will not be changed.

  • As isql starts multiple transactions for a single connection, ALTER SESSION RESET cannot be executed in isql.

Error Handling

Any error raised by ON DISCONNECT triggers aborts the session reset and leaves the session state unchanged.Such errors are reported using primary error code isc_session_reset_err (335545206) and error text "Cannot reset user session".

Any error raised after ON DISCONNECT triggers (including the ones raised by ON CONNECT triggers) aborts both the session reset and the connection itself.Such errors are reported using primary error code isc_ses_reset_failed (335545272) and error text "Reset of user session failed. Connection is shut down.".Subsequent operations on the connection (except detach) will fail with error isc_att_shutdown (335544856).

Debugging

SET DEBUG OPTION

Sets debug options

Syntax
SET DEBUG OPTION option-name = value
Table 1. Supported options
Option name Value Type Description

DSQL_KEEP_BLR

BOOLEAN

Stores statement BLR for retrieval with isc_info_sql_exec_path_blr_bytes and isc_info_sql_exec_path_blr_text.

SET DEBUG OPTION configures debug information for the current connection.

Warning

Debug options are closely tied to engine internals and their usage is discouraged if you do not understand how these internals are subject to change between versions.