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
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:
For metadata-optional connections, the client sets the
variable to control whether the server returns result set
metadata. Permitted values are
all metadata) and
NONE (return no metadata).
The default is
FULL, so even for
metadata-optional connections, the server by default returns
For connections that are not metadata-optional, setting
NONE produces an error.
To check whether a result set has metadata, the client calls the
function. This function returns
RESULTSET_METADATA_NONE to indicate that the
result set has full metadata or no metadata, respectively.
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
variable, the client can invoke
mysql_result_metadata() for each
result set to determine whether it has metadata.