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MySQL Restrictions and Limitations  /  Restrictions on Server-Side Cursors

Chapter 3 Restrictions on Server-Side Cursors

Server-side cursors are implemented in the C API using the mysql_stmt_attr_set() function. The same implementation is used for cursors in stored routines. A server-side cursor enables 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 an internal temporary table. Initially, this is a MEMORY table, but is converted to a MyISAM table when its size exceeds the minimum value of the max_heap_table_size and tmp_table_size system variables. The same restrictions apply to internal temporary tables created to hold the result set for a cursor as for other uses of internal temporary tables. See Internal Temporary Table Use in MySQL. 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 nonholdable (not held open after a commit).

Cursors are asensitive.

Cursors are nonscrollable.

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 TABLE, HANDLER READ, and SHOW BINLOG EVENTS.


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