184.108.40.206 The MySQL Diagnostics Area
SQL statements produce diagnostic information that populates the diagnostics area. Standard SQL has a diagnostics area stack, containing a diagnostics area for each nested execution context. Standard SQL also supports
GET STACKED DIAGNOSTICS syntax for referring to the second diagnostics area during condition handler execution. MySQL supports the
STACKED keyword as of MySQL 5.7. Before that, MySQL does not support
STACKED; there is a single diagnostics area containing information from the most recent statement that wrote to it.
The following discussion describes the structure of the diagnostics area in MySQL, the information items recognized by MySQL, how statements clear and set the diagnostics area, and how diagnostics areas are pushed to and popped from the stack.
The diagnostics area contains two kinds of information:
Statement information, such as the number of conditions that occurred or the affected-rows count.
Condition information, such as the error code and message. If a statement raises multiple conditions, this part of the diagnostics area has a condition area for each one. If a statement raises no conditions, this part of the diagnostics area is empty.
For a statement that produces three conditions, the diagnostics area contains statement and condition information like this:
Statement information: row count ... other statement information items ... Condition area list: Condition area 1: error code for condition 1 error message for condition 1 ... other condition information items ... Condition area 2: error code for condition 2: error message for condition 2 ... other condition information items ... Condition area 3: error code for condition 3 error message for condition 3 ... other condition information items ...
The diagnostics area contains statement and condition information items. Numeric items are integers. The character set for character items is UTF-8. No item can be
NULL. If a statement or condition item is not set by a statement that populates the diagnostics area, its value is 0 or the empty string, depending on the item data type.
The statement information part of the diagnostics area contains these items:
The condition information part of the diagnostics area contains a condition area for each condition. Condition areas are numbered from 1 to the value of the
NUMBER statement condition item. If
NUMBER is 0, there are no condition areas.
Each condition area contains the items in the following list. All items are standard SQL except
MYSQL_ERRNO, which is a MySQL extension. The definitions apply for conditions generated other than by a signal (that is, by a
RESIGNAL statement). For nonsignal conditions, MySQL populates only those condition items not described as always empty. The effects of signals on the condition area are described later.
CLASS_ORIGIN: A string containing the class of the
RETURNED_SQLSTATEvalue. If the
RETURNED_SQLSTATEvalue begins with a class value defined in SQL standards document ISO 9075-2 (section 24.1, SQLSTATE),
'ISO 9075'. Otherwise,
SUBCLASS_ORIGIN: A string containing the subclass of the
'ISO 9075'. Otherwise,
RETURNED_SQLSTATE: A string that indicates the
SQLSTATEvalue for the condition.
MESSAGE_TEXT: A string that indicates the error message for the condition.
MYSQL_ERRNO: An integer that indicates the MySQL error code for the condition.
CONSTRAINT_NAME: Strings that indicate the catalog, schema, and name for a violated constraint. They are always empty.
COLUMN_NAME: Strings that indicate the catalog, schema, table, and column related to the condition. They are always empty.
CURSOR_NAME: A string that indicates the cursor name. This is always empty.
MYSQL_ERRNO values for particular errors, see Section B.3.1, “Server Error Message Reference”.
RESIGNAL) statement populates the diagnostics area, its
SET clause can assign to any condition information item except
RETURNED_SQLSTATE any value that is legal for the item data type.
SIGNAL also sets the
RETURNED_SQLSTATE value, but not directly in its
SET clause. That value comes from the
SIGNAL also sets statement information items. It sets
NUMBER to 1. It sets
ROW_COUNT to -1 for errors and 0 otherwise.
Nondiagnostic SQL statements populate the diagnostics area automatically, and its contents can be set explicitly with the
RESIGNAL statements. The diagnostics area can be examined with
GET DIAGNOSTICS to extract specific items, or with
SHOW WARNINGS or
SHOW ERRORS to see conditions or errors.
SQL statements clear and set the diagnostics area as follows:
When the server starts executing a statement after parsing it, it clears the diagnostics area for nondiagnostic statements. Diagnostic statements do not clear the diagnostics area. These statements are diagnostic:
If a statement raises a condition, the diagnostics area is cleared of conditions that belong to earlier statements. The exception is that conditions raised by
RESIGNALare added to the diagnostics area without clearing it.
Thus, even a statement that does not normally clear the diagnostics area when it begins executing clears it if the statement raises a condition.
The following example shows the effect of various statements on the diagnostics area, using
SHOW WARNINGS to display information about conditions stored there.
DROP TABLE statement clears the diagnostics area and populates it when the condition occurs:
mysql> DROP TABLE IF EXISTS test.no_such_table; Query OK, 0 rows affected, 1 warning (0.01 sec) mysql> SHOW WARNINGS; +-------+------+------------------------------------+ | Level | Code | Message | +-------+------+------------------------------------+ | Note | 1051 | Unknown table 'test.no_such_table' | +-------+------+------------------------------------+ 1 row in set (0.00 sec)
SET statement generates an error, so it clears and populates the diagnostics area:
mysql> SET @x = @@x; ERROR 1193 (HY000): Unknown system variable 'x' mysql> SHOW WARNINGS; +-------+------+-----------------------------+ | Level | Code | Message | +-------+------+-----------------------------+ | Error | 1193 | Unknown system variable 'x' | +-------+------+-----------------------------+ 1 row in set (0.00 sec)
SET statement produced a single condition, so 1 is the only valid condition number for
GET DIAGNOSTICS at this point. The following statement uses a condition number of 2, which produces a warning that is added to the diagnostics area without clearing it:
mysql> GET DIAGNOSTICS CONDITION 2 @p = MESSAGE_TEXT; Query OK, 0 rows affected, 1 warning (0.00 sec) mysql> SHOW WARNINGS; +-------+------+------------------------------+ | Level | Code | Message | +-------+------+------------------------------+ | Error | 1193 | Unknown system variable 'xx' | | Error | 1753 | Invalid condition number | +-------+------+------------------------------+ 2 rows in set (0.00 sec)
Now there are two conditions in the diagnostics area, so the same
GET DIAGNOSTICS statement succeeds:
mysql> GET DIAGNOSTICS CONDITION 2 @p = MESSAGE_TEXT; Query OK, 0 rows affected (0.00 sec) mysql> SELECT @p; +--------------------------+ | @p | +--------------------------+ | Invalid condition number | +--------------------------+ 1 row in set (0.01 sec)
When a push to the diagnostics area stack occurs, the first (current) diagnostics area becomes the second (stacked) diagnostics area and a new current diagnostics area is created as a copy of it. Diagnostics areas are pushed to and popped from the stack under the following circumstances:
Execution of a stored program
A push occurs before the program executes and a pop occurs afterward. If the stored program ends while handlers are executing, there can be more than one diagnostics area to pop; this occurs due to an exception for which there are no appropriate handlers or due to
RETURNin the handler.
Any warning or error conditions in the popped diagnostics areas then are added to the current diagnostics area, except that, for triggers, only errors are added. When the stored program ends, the caller sees these conditions in its current diagonstics area.
Execution of a condition handler within a stored program
When a push occurs as a result of condition handler activation, the stacked diagnostics area is the area that was current within the stored program prior to the push. The new now-current diagnostics area is the handler's current diagnostics area.
GET [CURRENT] DIAGNOSTICSand
GET STACKED DIAGNOSTICScan be used within the handler to access the contents of the current (handler) and stacked (stored program) diagnostics areas. Initially, they return the same result, but statements executing within the handler modify the current diagnostics area, clearing and setting its contents according to the normal rules (see How the Diagnostics Area is Cleared and Populated). The stacked diagnostics area cannot be modified by statements executing within the handler except
If the handler executes successfully, the current (handler) diagnostics area is popped and the stacked (stored program) diagnostics area again becomes the current diagnostics area. Conditions added to the handler diagnostics area during handler execution are added to the current diagnostics area.
RESIGNALstatement passes on the error condition information that is available during execution of a condition handler within a compound statement inside a stored program.
RESIGNALmay change some or all information before passing it on, modifying the diagnostics stack as described in Section 220.127.116.11, “RESIGNAL Statement”.
Certain system variables control or are related to some aspects of the diagnostics area:
max_error_countcontrols the number of condition areas in the diagnostics area. If more conditions than this occur, MySQL silently discards information for the excess conditions. (Conditions added by
RESIGNALare always added, with older conditions being discarded as necessary to make room.)
warning_countindicates the number of conditions that occurred. This includes errors, warnings, and notes. Normally,
warning_countare the same. However, as the number of conditions generated exceeds
max_error_count, the value of
warning_countcontinues to rise whereas
NUMBERremains capped at
max_error_countbecause no additional conditions are stored in the diagnostics area.
error_countindicates the number of errors that occurred. This value includes “not found” and exception conditions, but excludes warnings and notes. Like
warning_count, its value can exceed
max_error_count is 10, the diagnostics area can contain a maximum of 10 condition areas. Suppose that a statement raises 20 conditions, 12 of which are errors. In that case, the diagnostics area contains the first 10 conditions,
NUMBER is 10,
warning_count is 20, and
error_count is 12.
Changes to the value of
max_error_count have no effect until the next attempt to modify the diagnostics area. If the diagnostics area contains 10 condition areas and
max_error_count is set to 5, that has no immediate effect on the size or content of the diagnostics area.