# SYSCS_DIAG diagnostic tables and functions

Derby provides a set of system table expressions which you can use to obtain diagnostic information about the state of the database and about the database sessions.

There are two types of diagnostic table expressions in Derby:

Diagnostic tables
  Tables that are like any other table in Derby. You can specify the diagnostic table name anywhere a normal table name is allowed.
Diagnostic table functions
  Functions that are like any other function in Derby. Diagnostic table functions can accept zero or more arguments, depending on the table function that you use. You must use the SQL-defined table function syntax to access these functions.

The following table shows the types and names of the diagnostic table expressions in Derby.

Table 1. System diagnostic table expressions provided by Derby
Diagnostic table expression | Type of expression
--- | ---
SYSCS_DIAG.ERROR_LOG_READER | Table function
SYSCS_DIAG.ERROR_MESSAGES | Table
SYSCS_DIAG.LOCK_TABLE | Table
SYSCS_DIAG.SPACE_TABLE | Table function
SYSCS_DIAG.STATEMENT_CACHE | Table
SYSCS_DIAG.STATEMENT_DURATION | Table function
SYSCS_DIAG.TRANSACTION_TABLE | Table

Restriction: If you reference a diagnostic table in a DDL statement or a compression procedure, Derby returns an exception.

#### SYSCS_DIAG.ERROR_LOG_READER diagnostic table function

The SYSCS_DIAG.ERROR_LOG_READER diagnostic table function contains all the useful SQL statements that are in the derby.log file or a log file that you specify.

One use of this diagnostic table function is to determine the active transactions and the SQL statements in those transactions at a given point in time. For example, if a deadlock or lock timeout occurred you can find the timestamp (timestampConstant) in the error log.

To access the SYSCS_DIAG.ERROR_LOG_READER diagnostic table function, you must use the SQL table function syntax.

For example:

    SELECT *
        FROM TABLE (SYSCS_DIAG.ERROR_LOG_READER())
        AS T1

where T1 is a user-specified table name that is any valid identifier.

You can specify a log file name as an optional argument to the SYSCS_DIAG.ERROR_LOG_READER diagnostic table function. When you specify a log file name, the file name must be an expression whose data type maps to a Java string.

For example:

    SELECT *
        FROM TABLE (SYSCS_DIAG.ERROR_LOG_READER('myderbyerrors.log'))
        AS T1

Tip: By default Derby log files contain only boot, shutdown, and error messages. In the Tuning Java DB guide, see the derby.stream.error.logSeverityLevel property and the derby.language.logStatementText property for instructions on how to print more information to Derby log files. You can then query that information by using the SYSCS_DIAG.ERROR_LOG_READER diagnostic table function.

#### SYSCS_DIAG.ERROR_MESSAGES diagnostic table

The SYSCS_DIAG.ERROR_MESSAGES diagnostic table shows all of the SQLStates, locale-sensitive error messages, and exception severities for a Derby database. You can reference the SYSCS_DIAG.ERROR_MESSAGES diagnostic table directly in a statement.

For example:

    SELECT * FROM SYSCS_DIAG.ERROR_MESSAGES

#### SYSCS_DIAG.LOCK_TABLE diagnostic table

The SYSCS_DIAG.LOCK_TABLE diagnostic table shows all of the locks that are currently held in the Derby database. You can reference the SYSCS_DIAG.LOCK_TABLE diagnostic table directly in a statement.

For example:

    SELECT * FROM SYSCS_DIAG.LOCK_TABLE

When the SYSCS_DIAG.LOCK_TABLE diagnostic table is referenced in a statement, a snap shot of the lock table is taken. A snap shot is used so that referencing the diagnostic table does not alter the normal timing and flow of the application. It is possible that some locks will be in a transition state when the snap shot is taken.

#### SYSCS_DIAG.SPACE_TABLE diagnostic table function

The SYSCS_DIAG.SPACE_TABLE diagnostic table function shows the space usage of a particular table and its indexes. You can use this diagnostic table function to determine if space might be saved by compressing the table and indexes.

To access the SYSCS_DIAG.SPACE_TABLE diagnostic table function, you must use the SQL table function syntax. This diagnostic table function takes two arguments, the schemaName and the tableName. The tableName argument is required. If you do not specify the schemaName, the current schema is used.

For example, use the following query to return the space usage for all of the user tables and indexes in the database:

    SELECT T2.*
        FROM
            SYS.SYSTABLES systabs,
            TABLE (SYSCS_DIAG.SPACE_TABLE(systabs.tablename)) AS T2
        WHERE systabs.tabletype = 'T'

where T2 is a user-specified table name that is any valid identifier.

Both the schemaName and the tableName arguments must be expressions whose data types map to Java strings. If the schemaName and the tableName are non-delimited identifiers, you must specify the names in upper case.

For example:

    SELECT *
        FROM TABLE (SYSCS_DIAG.SPACE_TABLE('MYSCHEMA', 'MYTABLE'))
        AS T2

#### SYSCS_DIAG.STATEMENT_CACHE diagnostic table

The SYSCS_DIAG.STATEMENT_CACHE diagnostic table shows the contents of the SQL statement cache. You can reference the SYSCS_DIAG.STATEMENT_CACHE diagnostic table directly in a statement.

For example:

    SELECT * FROM SYSCS_DIAG.STATEMENT_CACHE

#### SYSCS_DIAG.STATEMENT_DURATION diagnostic table function

You can use the SYSCS_DIAG.STATEMENT_DURATION diagnostic table function to analyze the execution duration of the useful SQL statements in the derby.log file or a log file that you specify.

You can also use this diagnostic table function to get an indication of where the bottlenecks are in the JDBC code for an application.

To access the SYSCS_DIAG.STATEMENT_DURATION diagnostic table function, you must use the SQL table function syntax.

For example:

    SELECT *
        FROM TABLE (SYSCS_DIAG.STATEMENT_DURATION())
        AS T1

where T1 is a user-specified table name that is any valid identifier.
Restriction: For each transaction ID, a row is not returned for the last statement with that transaction id. Transaction IDs change within a connection after a commit or rollback, if the transaction that just ended modified data.

You can specify a log file name as an optional argument to the SYSCS_DIAG.STATEMENT_DURATION diagnostic table function. When you specify a log file name, the file name must be an expression whose data type maps to a Java string.

For example:

    SELECT *
        FROM TABLE (SYSCS_DIAG.STATEMENT_DURATION('somederby.log'))
        AS T1

Tip: By default Derby log files contain only boot, shutdown, and error messages. In the Tuning Java DB guide, see the derby.stream.error.logSeverityLevel property and the derby.language.logStatementText property for instructions on how to print more information to Derby log files. You can then query that information by using the SYSCS_DIAG.STATEMENT_DURATION diagnostic table function.

#### SYSCS_DIAG.TRANSACTION_TABLE diagnostic table

The SYSCS_DIAG.TRANSACTION_TABLE diagnostic table shows all of the transactions that are currently in the database. You can reference the SYSCS_DIAG.TRANSACTION_TABLE diagnostic table directly in a statement.

For example:

    SELECT * FROM SYSCS_DIAG.TRANSACTION_TABLE

When the SYSCS_DIAG.TRANSACTION_TABLE diagnostic table is referenced in a statement, a snap shot of the transaction table is taken. A snap shot is used so that referencing the diagnostic table does not alter the normal timing and flow of the application. It is possible that some transactions will be in a transition state when the snap shot is taken.

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