3.6.7 Optional Result Set Metadata

When a client executes a statement that produces a result set, MySQL makes available the data the result set contains, and by default also result set metadata that provides information about the result set data. Metadata is contained in the MYSQL_FIELD structure (see Section 5.2, “C API Basic Data Structures”), which is returned by the mysql_fetch_field(), mysql_fetch_field_direct(), and mysql_fetch_fields() functions.

Clients can indicate on a per-connection basis that result set metadata is optional and that the client will indicate to the server whether to return it. Suppression of metadata transfer by the client can improve performance, particularly for sessions that execute many queries that return few rows each.

There are two ways for a client to indicate that result set metadata is optional for a connection. They are equivalent, so either one suffices:

  • Prior to connect time, enable the MYSQL_OPT_OPTIONAL_RESULTSET_METADATA option for mysql_options().

  • At connect time, enable the CLIENT_OPTIONAL_RESULTSET_METADATA flag for the client_flag argument of mysql_real_connect().

For metadata-optional connections, the client sets the resultset_metadata system variable to control whether the server returns result set metadata. Permitted values are FULL (return all metadata) and NONE (return no metadata). The default is FULL, so even for metadata-optional connections, the server by default returns metadata.

For metadata-optional connections, the mysql_fetch_field(), mysql_fetch_field_direct(), and mysql_fetch_fields() functions return NULL when resultset_metadata is set to NONE.

For connections that are not metadata-optional, setting resultset_metadata to NONE produces an error.

To check whether a result set has metadata, the client calls the mysql_result_metadata() function. This function returns RESULTSET_METADATA_FULL or RESULTSET_METADATA_NONE to indicate that the result set has full metadata or no metadata, respectively.

mysql_result_metadata() is useful if the client does not know in advance whether a result set has metadata. For example, if a client executes a stored procedure that returns multiple result sets and might change the resultset_metadata system variable, the client can invoke mysql_result_metadata() for each result set to determine whether it has metadata.