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Overloading Procedures

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A student asked, “Can you successfully overload MySQL stored procedures, like PL/SQL does in stored packages?” I said, “MySQL doesn’t formally support overloading stored procedures, and doesn’t support packages like Oracle 11g. You can, however, mimic (or fake) overloading with prepared statements. It requires a careful combination of stored procedures and session variables.”

Unfortunately, they asked for an example. So I wrote this set of code. It uses queries as the dynamic statements to make it simple to test but you can substitute INSERT, UPDATE, or DELETE statements. I didn’t provide type validation in the example, which would be required for dates or date-timestamps.

It’s presented in steps with test programs at each level. If you see an opportunity to improve on any of the example components, leave a comment. As to whether I’d implement this in production code, the answer is no but I believe all possibilities should at least be explored.

Step #1

The first step requires defining a stored procedure that sets a session variable. It’s fairly straight forward, but remember to change those DELIMITER values when testing. (As a note, you don’t require a semicolon after the END statement of a stored function or procedure in MySQL. You only need the redefined DELIMITER, which is $$ in these examples.)

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DELIMITER $$
 
-- Drop the procedure if it exists.
DROP PROCEDURE IF EXISTS set_session_var$$
 
-- Create the procedure.
CREATE PROCEDURE set_session_var
( pv_session_name   VARCHAR(32767)
, pv_session_value  VARCHAR(32767))
CONTAINS SQL
BEGIN
 
  /* Insert statement with auto commit enabled. */
  SET @sql := concat('SET',' ','@',pv_session_name,' := ','?');
  SELECT @sql AS "SQL String";
  PREPARE stmt FROM @sql;
  SET @sv_session_value := pv_session_value;
  EXECUTE stmt using @sv_session_value;
  DEALLOCATE PREPARE stmt;
 
END;
$$

The test case for the set_session_var is:

CALL set_session_var('sv_filter1','One');
CALL set_session_var('sv_filter2','Two');
SELECT @sv_filter1, @sv_filter2;

Important note: If you call this from another stored program you can’t use the stmt variable name in the calling program’s scope.

Step #2

The second step exists because you can’t pass arrays in MySQL (a restriction that also exists for T-SQL in Microsoft SQL Server). You have to pass a varying list of parameters as a serialized string. This is often called flexible parameter passing, which many PHP programmers leverage outside the database (flexible parameter passing is covered in this PHP tutorial I wrote a while back).

The deserialize example does three things:

  • Takes a serialized set of parameters and deserializes the parameters into a set of name-value pairs, which are stored by their name as session level variables.
  • It records the number of name-value pairs written as session variables in a temporary table. The use of a temporary table is required because a MySQL stored function can’t hold a dynamic statement or a call to a procedure with a dynamic statement.
  • The script checks for the existence of the table during execution and truncates it when found. It creates the table when it isn’t found. While it would be ideal to use a temporary table, they’re not disclosed in the the information_schema and therefore more tedious to manage. Creating the table in the MEMORY database avoids fragmenting the InnoDB tablespace.

Here’s the working code:

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CREATE PROCEDURE deserialize
( pv_param_list VARCHAR(32767))
CONTAINS SQL
BEGIN
  DECLARE lv_name     VARCHAR(9) DEFAULT 'sv_filter';
  DECLARE lv_length   INT;
  DECLARE lv_start    INT DEFAULT 1;
  DECLARE lv_end      INT DEFAULT 1;
  DECLARE lv_counter  INT DEFAULT 1;
 
  /* Skip when call parameter list is null or empty. */	
  IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN
 
    /* Read line by line on a line return character. */
    parse: WHILE NOT (lv_end = 0) DO
 
      /* Check for line returns. */
      SET lv_end := LOCATE(',',pv_param_list,lv_start);
 
      /* Check whether line return has been read. */
      IF NOT lv_end = 0 THEN  /* Reset the ending substring value. */
        SET lv_end := LOCATE(',',pv_param_list,lv_start);
        CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start));
      ELSE  /* Print the last substring with a semicolon. */
        CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list)));
      END IF;
 
      /* Reset the beginning of the string. */
      SET lv_start := lv_end + 1;      
      SET lv_counter := lv_counter + 1;
 
    END WHILE parse;    
 
  END IF;
 
  /* Check for a temporary table that holds a control variable,
     create the table if it doesn't exist, and remove rows from
     the table. */
  IF EXISTS (SELECT   null
             FROM     information_schema.tables
             WHERE    table_name = 'counter') THEN
    TRUNCATE TABLE counter;
  ELSE
    /* It would be ideal to use a temporary table here but then
       it's not recorded in the INFORMATION_SCHEMA and cleansing
       the temporary table is more tedious. */
    CREATE TABLE counter ( counter INT ) ENGINE=MEMORY;
  END IF;
 
  /* Insert the counter value for a list of parameters. */
  INSERT INTO counter VALUES ( lv_counter - 1 );  
 
END;
$$

Bill Karwin made a wonderful point about the overhead of looking up a table name and using a table rather than a temporary table. You can replace the logic above with a CONTINUE HANDLER and a temporary table. It should resolve the problems mentioned in the note but as mentioned earlier there are other solutions for production systems.

Bill also referenced a great post by Jay Pipes. It points out that there are serious problems scaling MySQL stored programs. This occurs because MySQL stored procedures are not compiled and stored in a global stored procedure cache, like Microsoft SQL Server or Oracle.

This deficit in MySQL certainly plays to the philosophy of developing everything in the application layer, suggested by Bill. At least, that is true when the application layer is co-resident on the server-tier as a Server Side Includes (SSI) library.

It does beg the question why Microsoft SQL Server and Oracle enable placing an efficient set of stored logic in the database server, effectively creating an application layer that can be split between an SSI library and the database server. After all, the server has been the most scalable component and the application layer can distribute to the middle-tier.

Today’s limitations on MySQL stored programs present a clear and compelling case toward their general avoidance and specific use for delivering a scalable solutions. Will that hold true beyond 2015 when licensing becomes completely proprietary to Oracle? I’d venture (a complete guess or hope on my part) a global stored procedure cache may be the future of MySQL procedures. It would certainly serve to distinguish MySQL from MariaDB and provide an incentive to remain on MySQL or to return from MariaDB to MySQL. Such a change would certainly make MySQL even more competitive against Microsoft SQL Server, which is the only rival purely relational database it competes against.

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CREATE PROCEDURE deserialize
( pv_param_list VARCHAR(32767))
CONTAINS SQL
BEGIN
  DECLARE lv_name     VARCHAR(9) DEFAULT 'sv_filter';
  DECLARE lv_length   INT;
  DECLARE lv_start    INT DEFAULT 1;
  DECLARE lv_end      INT DEFAULT 1;
  DECLARE lv_counter  INT DEFAULT 1;
 
  DECLARE CONTINUE HANDLER FOR 1146
    BEGIN
      /* Create a temporary table. */
      CREATE TEMPORARY TABLE counter ( counter INT ) ENGINE=MEMORY;
    END;
 
  /* Skip when call parameter list is null or empty. */	
  IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN
 
    /* Read line by line on a line return character. */
    parse: WHILE NOT (lv_end = 0) DO
 
      /* Check for line returns. */
      SET lv_end := LOCATE(',',pv_param_list,lv_start);
 
      /* Check whether line return has been read. */
      IF NOT lv_end = 0 THEN  /* Reset the ending substring value. */
        SET lv_end := LOCATE(',',pv_param_list,lv_start);
        CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start));
      ELSE  /* Print the last substring with a semicolon. */
        CALL set_session_var(CONCAT(lv_name,lv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list)));
      END IF;
 
      /* Reset the beginning of the string. */
      SET lv_start := lv_end + 1;      
      SET lv_counter := lv_counter + 1;
 
    END WHILE parse;    
 
  END IF;
 
  /* Truncate existing table. */
  TRUNCATE TABLE counter;
 
  /* Insert the counter value for a list of parameters. */
  INSERT INTO counter VALUES ( lv_counter - 1 );  
 
END;
$$

The test program for deserialize follows below. If you’re curious, those are the Italian version of one through four. 😉 I’ve a partiality for Italian after living in Italy for two years.

CALL deserialize('Uno,Due,Tre,Quattro');
SELECT counter AS "Parameter #" FROM counter;
SELECT @sv_filter1, @sv_filter2, @sv_filter3, @sv_filter4;

You can also eliminate the temporary table completely by replacing the counter with a @sv_counter session variable, as mentioned by Bill in his comment. The following is the version of deserialize uses a session variable.

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CREATE PROCEDURE deserialize
( pv_param_list VARCHAR(32767))
CONTAINS SQL
BEGIN
  DECLARE lv_name     VARCHAR(9) DEFAULT 'sv_filter';
  DECLARE lv_length   INT;
  DECLARE lv_start    INT DEFAULT 1;
  DECLARE lv_end      INT DEFAULT 1;
 
  /* Set a session variable to enable a calling scope to
     read it without a function return. */
  SET @sv_counter := 1;
 
  /* Skip when call parameter list is null or empty. */	
  IF NOT (ISNULL(pv_param_list) OR LENGTH(pv_param_list) = 0) THEN
 
    /* Read line by line on a line return character. */
    parse: WHILE NOT (lv_end = 0) DO
 
      /* Check for line returns. */
      SET lv_end := LOCATE(',',pv_param_list,lv_start);
 
      /* Check whether line return has been read. */
      IF NOT lv_end = 0 THEN  /* Reset the ending substring value. */
        SET lv_end := LOCATE(',',pv_param_list,lv_start);
        CALL set_session_var(CONCAT(lv_name,@sv_counter),SUBSTR(pv_param_list,lv_start,lv_end - lv_start));
      ELSE  /* Print the last substring with a semicolon. */
        CALL set_session_var(CONCAT(lv_name,@sv_counter),SUBSTR(pv_param_list,lv_start,LENGTH(pv_param_list)));
      END IF;
 
      /* Reset the beginning of the string. */
      SET lv_start := lv_end + 1;      
      SET @sv_counter := @sv_counter + 1;
 
    END WHILE parse;    
 
  END IF;
 
  /* Reduce by one for 1-based numbering of name elements. */
  SET @sv_counter := @sv_counter - 1;
 
END;
$$

The test case changes for this version because there isn’t a table involved. You would use the following:

CALL deserialize('Uno,Due,Tre,Quattro');
SELECT @sv_filter1, @sv_filter2, @sv_filter3, @sv_filter4;

Like the earlier procedure, there’s no exception handling but it wouldn’t be a bad idea to add it. Especially, if you’re using the new SIGNAL feature of MySQL 5.6, which is now backported to MySQL 5.5.

Step #3

The last step creates a store procedure that takes two parameters, a parameterized statement and a serialized list of parameters. This means you can build any statement on the fly as required. The example uses queries simply because they’re the easiest to demonstrate the process.

You should note that the prepared statement is dynamic_stmt not stmt because stmt is used inside the set_session_var procedure. That means using stmt in the calling scope program would impact the called scope program because they use the same identifier (more or less a namespace scope issue).

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CREATE PROCEDURE prepared_dml
( pv_query   VARCHAR(32767)
, pv_filter  VARCHAR(32767))
CONTAINS SQL
BEGIN
 
  /* Declare a local variable for the SQL statement. */
  DECLARE dynamic_stmt  VARCHAR(32767);
  DECLARE lv_counter    INT DEFAULT 0;
 
  /* Cleanup the message passing table when a case is not found. */
  DECLARE EXIT HANDLER FOR 1339
    BEGIN
      /* Step #5: */
      DEALLOCATE PREPARE dynamic_stmt;
 
      /* Cleanup the temporary table that exchanges data between
         procedures. */
      DROP TABLE IF EXISTS counter;
    END;
 
  /* Step #1:
     ========
     Set a session variable with two parameter markers. */
  SET @SQL := pv_query;
 
  /* Verify query is not empty. */
  IF NOT ISNULL(@SQL) THEN
 
    /* Step #2:
       ========
       Dynamically allocated and run statement. */
    PREPARE dynamic_stmt FROM @SQL;
 
    /* Step #3:
       ========
       Assign the formal parameters to session variables
       because prepared statements require them. */
    CALL deserialize(pv_filter);
 
    /* Secure the parameter count from a temporary table that
       exchanges data between procedures. */
    SELECT counter INTO lv_counter FROM counter;
 
    /* Step #4:
       ========
       Choose the appropriate overloaded prepared statement. */
    CASE
      WHEN lv_counter = 0 THEN
        EXECUTE dynamic_stmt;      
      WHEN lv_counter = 1 THEN
        EXECUTE dynamic_stmt USING @sv_filter1;
      WHEN lv_counter = 2 THEN
        EXECUTE dynamic_stmt USING @sv_filter1,@sv_filter2;
    END CASE;
 
    /* Step #5: */
    DEALLOCATE PREPARE dynamic_stmt;
 
    /* Cleanup the temporary table that exchanges data between
       procedures. */
    DROP TABLE IF EXISTS counter; 
 
  END IF;
 
END;
$$

If you’re using the deserialize procedure that uses a temporary table, you should make the following changes prepared_dml procedure on line 19 and 62. They modify the DROP TABLE statement to a DROP TEMPORARY TABLE statement. Though the change isn’t technically required because a DROP TABLE works against temporary tables as well as ordinary tables, it generally clearer to those new to MySQL syntax.

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    /* Cleanup the temporary table that exchanges data between
       procedures. */
    DROP TEMPORARY TABLE IF EXISTS counter;

The line 62 change:

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    /* Cleanup the temporary table that exchanges data between
       procedures. */
    DROP TEMPORARY TABLE IF EXISTS counter;

It would be much nicer to avoid the CASE statement or an if-else-if block but prepared statements are limited on what SQL statements they support. For example, you can create or drop stored functions or procedures, and you can’t dispatch a dynamic statement as a dynamic statement. That means you can’t replace the CASE statement with something like this:

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    SET @dsql := 'EXECUTE dynamic_stmt USING @sv_filter1, @sv_filter2';
    PREPARE preparsed_stmt FROM @dsql;
    EXECUTE preparsed_stmt;
    DEALLOCATE PREPARE preparsed_stmt;

Attempt that, and you’ll get the following message. Does that mean there’s hope it’ll become a feature and possibility? Or, that it isn’t recommended?

ERROR 1295 (HY000): This command is not supported in the prepared statement protocol yet

The session variable solution requires another version of the prepared_dml procedure, as shown below. As you can imagine, removing the table and replacing a session variable simplifies the prepared_dml stored procedure version.

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CREATE PROCEDURE prepared_dml
( pv_query   VARCHAR(32767)
, pv_filter  VARCHAR(32767))
CONTAINS SQL
BEGIN
 
  /* Declare a local variable for the SQL statement. */
  DECLARE dynamic_stmt  VARCHAR(32767);
  DECLARE lv_counter    INT DEFAULT 0;
 
  /* Cleanup the message passing table when a case is not found. */
  DECLARE EXIT HANDLER FOR 1339
    BEGIN
      /* Step #5: */
      DEALLOCATE PREPARE dynamic_stmt;
    END;
 
  /* Step #1:
     ========
     Set a session variable with two parameter markers. */
  SET @SQL := pv_query;
 
  /* Verify query is not empty. */
  IF NOT ISNULL(@SQL) THEN
 
    /* Step #2:
       ========
       Dynamically allocated and run statement. */
    PREPARE dynamic_stmt FROM @SQL;
 
    /* Step #3:
       ========
       Assign the formal parameters to session variables
       because prepared statements require them. */
    CALL deserialize(pv_filter);
 
     /* Step #4:
       ========
       Choose the appropriate overloaded prepared statement. */
    CASE
      WHEN @sv_counter = 0 THEN
        EXECUTE dynamic_stmt;      
      WHEN @sv_counter = 1 THEN
        EXECUTE dynamic_stmt USING @sv_filter1;
      WHEN @sv_counter = 2 THEN
        EXECUTE dynamic_stmt USING @sv_filter1,@sv_filter2;
    END CASE;
 
    /* Step #5: */
    DEALLOCATE PREPARE dynamic_stmt;
 
  END IF;
 
END;
$$

The test programs check without a parameter and with one or two parameters, as you can see below.

SELECT 'Test Case #1 ...' AS "Statement";
SET @param1 := 'SELECT "Hello World"';
SET @param2 := '';
CALL prepared_dml(@param1,@param2);
 
SELECT 'Test Case #2 ...' AS "Statement";
SET @param1 := 'SELECT item_title FROM item i WHERE item_title REGEXP ?';
SET @param2 := '^.*war.*$';
CALL prepared_dml(@param1,@param2);
 
SELECT 'Test Case #3 ...' AS "Statement";
SET @param1 := 'SELECT common_lookup_type FROM common_lookup cl WHERE common_lookup_table REGEXP ? AND common_lookup_column REGEXP ?';
SET @param2 := 'item,item_type';
CALL prepared_dml(@param1,@param2);

As always, I hope this helps those writing MySQL Stored Procedures. If you’re new to stored programs, you can find a chapter on writing them in my Oracle Database 11g & MySQL 5.6 Developer Handbook or you can use Guy Harrison’s MySQL Stored Procedure Programming.

Written by maclochlainn

June 16th, 2012 at 7:36 pm

Posted in MySQL,sql,Stored Procedures

Tagged with , ,

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