Question

While pondering a potential solution to another question on SO, I started considering using a prepared statement within a procedure (no dynamic SQL in functions makes me sad). The naive approach of executing a PREPARE in the procedure seems inefficient, as the statement only needs to be prepared the first time the procedure is called during a session. Is it possible in MySQL to prepare a statement only if it doesn’t already exist? Is there a way of checking for the existence of a prepared statement?

Here’s a sample procedure for people to play with. It creates a statement that SELECTs all columns of the given type from the given table.

DROP PROCEDURE IF EXISTS select_columns_of_type_statement;
delimiter //
CREATE PROCEDURE select_columns_of_type_statement(IN db VARCHAR(255), IN tbl VARCHAR(255), IN type VARCHAR(255), OUT result VARCHAR(4095))
  READS SQL DATA
  NOT DETERMINISTIC -- as tables can be ALTERed
BEGIN
    SET @scots_db=db, @scots_tbl=tbl, @scots_type=type;
    PREPARE generate_scots FROM "SELECT CONCAT(
          'SELECT ',
          GROUP_CONCAT(COLUMN_NAME SEPARATOR ', '),
          ' FROM `', ?, '`.`', ?, '` '
          )
      FROM INFORMATION_SCHEMA.COLUMNS
      WHERE TABLE_SCHEMA=? AND TABLE_NAME=? AND data_type=?
      INTO @scots_stmt";
    EXECUTE generate_scots USING @scots_db, @scots_tbl, @scots_db, @scots_tbl, @scots_type;
    SET result = @scots_stmt;
END //
delimiter ;

Example usage:

CALL select_columns_of_type_statement('mysql', 'user', 'int', @stmt);
SELECT @stmt;
-- or even:
SET @stmt=CONCAT(@stmt, 'LIMIT 3');
PREPARE user_int_cols FROM @stmt;
EXECUTE user_int_cols;

That user variables must be used so much also makes me sad.

Answer 1

Checking existence may not be possible, but the try-and-handle-failure pattern will work. DECLARE an error handler that defines & executes the prepared statement, and execute the prepared statement outside of the handler. If the statement doesn’t exist, the error handler will run. It’s not pretty, but it works.

DROP PROCEDURE IF EXISTS select_columns_of_type_statement;
delimiter //
CREATE PROCEDURE select_columns_of_type_statement(IN db VARCHAR(255), IN tbl VARCHAR(255), IN type VARCHAR(255), OUT result VARCHAR(4095))
  READS SQL DATA
  NOT DETERMINISTIC -- as tables can be ALTERed
BEGIN
    DECLARE CONTINUE HANDLER
      FOR 1243
      BEGIN
        PREPARE generate_scots FROM "SELECT CONCAT(
              'SELECT ',
              GROUP_CONCAT(COLUMN_NAME SEPARATOR ', '),
              ' FROM `', ?, '`.`', ?, '` '
              )
          FROM INFORMATION_SCHEMA.COLUMNS
          WHERE TABLE_SCHEMA=? AND TABLE_NAME=? AND data_type=?
          INTO @scots_stmt";
        EXECUTE generate_scots USING @scots_db, @scots_tbl, @scots_db, @scots_tbl, @scots_type;
      END;
    SET @scots_db=db, @scots_tbl=tbl, @scots_type=type;
    EXECUTE generate_scots USING @scots_db, @scots_tbl, @scots_db, @scots_tbl, @scots_type;
    SET result = @scots_stmt;
END //
delimiter ;