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F.2. Restrictions on Server-Side Cursors

Server-side cursors are implemented beginning with the C API in MySQL 5.0.2 via the mysql_stmt_attr_set() function. A server-side cursor allows a result set to be generated on the server side, but not transferred to the client except for those rows that the client requests. For example, if a client executes a query but is only interested in the first row, the remaining rows are not transferred.

In MySQL, a server-side cursor is materialized into a temporary table. Initially, this is a MEMORY table, but is converted to a MyISAM table if its size reaches the value of the max_heap_table_size system variable. (Beginning with MySQL 5.0.14, the same temporary-table implementation also is used for cursors in stored routines.) One limitation of the implementation is that for a large result set, retrieving its rows through a cursor might be slow.

Cursors are read-only; you cannot use a cursor to update rows.

UPDATE WHERE CURRENT OF and DELETE WHERE CURRENT OF are not implemented, because updatable cursors are not supported.

Cursors are non-holdable (not held open after a commit).

Cursors are asensitive.

Cursors are non-scrollable.

Cursors are not named. The statement handler acts as the cursor ID.

You can have open only a single cursor per prepared statement. If you need several cursors, you must prepare several statements.

You cannot use a cursor for a statement that generates a result set if the statement is not supported in prepared mode. This includes statements such as CHECK TABLES, HANDLER READ, and SHOW BINLOG EVENTS.


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