WL#3433: Stored Procedures: Dynamic Cursors
Affects: Server-7.1 — Status: Un-Assigned — Priority: Very High
Well-supported dynamic cursors have four characteristics: 1. Result Set from a 'SELECT' in a variable, not a SELECT that's hard-coded in the DECLARE CURSOR statement. 2. Metadata about the number of columns and their data types. 3. Variable definition of variables, that is, "DECLARE variable data_type" statements (or something equivalent), for as many variables as there are in the 'SELECT' variable, with declared-variable data types the same as data types of the columns of the 'SELECT' result set. 4. FETCH statements that bring in exactly the number of columns in the 'SELECT' result set, instead of the fixed number that MySQL has now with "FETCH c INTO v1,v2,v3;".
Here are four different proposals. All accomplish the same thing, approximately. Feel free to add more. Proposal #1: Do Nothing ----------------------- Users can get the "dynamic cursor" effects with temporary tables. To create the result set: use a CREATE TEMPORARY TABLE ... AS SELECT. To view the metadata: use information_schema with table_name = temporary table name. To define variables: DECLARE, in advance, as many variables as could possibly be necessary for a SELECT, with a generic data type like VARCHAR (just depend on MySQL's automatic data type conversion if the actual data type is not VARCHAR). Example: CREATE PROCEDURE p1 (select_statement VARCHAR(255)) BEGIN DECLARE v1,v2 VARCHAR(255); DECLARE c CURSOR FOR SELECT * FROM t; SET @v = CONCAT('create temporary table t as ',select_statement); PREPARE stmt1 FROM @v; EXECUTE stmt1; OPEN c; FETCH c INTO v1,v2; SELECT v1,v2; END// Actually this works well enough that we can propose it as a workaround for people who complain about the lack of support for dynamic cursors. And perhaps all the other Proposals will merely be new syntaxes which hide the use of a temporary table. The disadvantages are: - The temporary table is not local to the procedure, so two different procedures must use different names, and two invocations of the same procedure must avoid "table already exists" messages. - The 'SELECT' variable (@v in the example) is not local to the procedure either, although there is an intent to fix that (WL#2793). - The FETCH is still fixed, so one would have to add NULLs to the SELECT list so that the number of columns in the result set will equal the number to be FETCHed. - not all SHOW statements have an equivalent INFORMATION_SCHEMA. Currently there is no way to retrieve their result set via a cursor. Proposal #2: Rowtype -------------------- We can copy the Oracle %ROWTYPE, which is (I quote the Oracle manual here) for "a record type that represents a row ... from a previously declared cursor. Fields in the record and corresponding columns in the row have the same names and datatypes." WL#2954, in raw-idea bin, mentions %ROWTYPE. Our stored-procedure syntax differs from Oracle's, so the following is a hypothetical example using %ROWTYPE with MySQL syntax, not Oracle syntax. CREATE PROCEDURE p2 () BEGIN DECLARE cursor1 CURSOR FOR SELECT c1,c2 FROM t; BEGIN DECLARE v cursor1.%ROWTYPE; OPEN cursor1; FETCH cursor1 INTO v; SELECT v.c2; END; END// Here, v is a row variable which has two parts, c1 and c2. Their names and definitions come from the names and definitions in the cursor's SELECT list. In the example, v can be a qualifier, so "SELECT v.c1" means "SELECT the value that was just FETCHed for the c2 column in the cursor's SELECT list". The disadvantages are: - %ROWTYPE is non-standard - our %ROWTYPE isn't very Oracle-like either, although Oracle users should understand what we're doing immediately - we don't have the metadata for v, probably it would be best to "extend" by allowing references to ordinal numbers, for example 'v.' is the same as 'v.c2'. Proposal #3: Allocations ------------------------ We can do the cursors and variable declarations with dynamic ALLOCATE statements, as opposed to DECLAREs. This is vaguely like what happens with embedded SQL, and is perhaps closest to the standard (SQL:2003 non-core feature B031 "Basic dynamic SQL" for module language). The important new statements are ALLOCATE CURSOR, ALLOCATE DESCRIPTOR, DESCRIBE, and GET. Example: CREATE PROCEDURE p3 () BEGIN DECLARE v1 INT; DECLARE v2 CHAR(64) CHARACTER SET UTF8; PREPARE stmt1 FROM 'SELECT 5 AS a'; ALLOCATE c CURSOR FOR stmt1; ALLOCATE DESCRIPTOR d; DESCRIBE OUTPUT stmt1 USING DESCRIPTOR d; OPEN c; FETCH c USING d; GET DESCRIPTOR d v1 = CARDINALITY; GET DESCRIPTOR d VALUE 5 v2 = NAME; CLOSE c; END// ALLOCATE CURSOR will create a cursor. It's not DECLARE so it doesn't have to be right at the procedure start. Therefore it can follow PREPARE, without requiring a separate BEGIN ... END like the one in Proposal#1. ALLOCATE DESCRIPTOR will create a space for the descriptor. That is: it's just a malloc. DESCRIBE OUTPUT will fill the descriptor with the metadata for the prepared statement. I used "DESCRIBE OUTPUT stmt1" in the example but I could just as easily have said "DESCRIBE OUTPUT c", that is, I could use either the prepared statement name or the cursor name. FETCH will bring in the data, but not directly into declared variables. Instead, think of the descriptor as having pointers to the data as a result of the FETCH. Now the descriptor has both metadata and pointers to data. There are many descriptor fields, we can access them with GET statements. "GET DESCRIPTOR ... CARDINALITY" will place the "number of columns in the result set" into the declared variable v1. "GET DESCRIPTOR ... VALUE 5 ... NAME" will place the "name of the fifth column in the result set" into the declared variable v2. The disadvantages are: - This can be complicated, which may not be obvious because I've left out all the details. - The technique is more commonly used in embedded SQL than in stored procedures, and I think Monty dislikes things that look like embedded SQL. Proposal #4: User-Defined Type ------------------------------ This is a variant of Proposal#2. We can play with syntax that has %ROWTYPE's effect, but looks more like standard syntax than Oracle syntax. Example: CREATE PROCEDURE p4 () BEGIN DECLARE c CURSOR FOR stmt1; BEGIN CREATE TYPE t FROM c; BEGIN DECLARE v t; OPEN c; FETCH c INTO v; END; END; END// Here, we're creating a new data type in an executable statement ("CREATE TYPE ..."), and it's a row data type, so we can use it just like we used cursor1.%ROWTYPE for the 'v' variable in Proposal#2. Disadvantages: - Although CREATE TYPE looks like standard SQL, this particular example isn't. - People might actually prefer the %ROWTYPE syntax. References ---------- Other worklog entries: WL#2793 Prepared Statements: convenient SQL syntax for use in Dynamic SQL WL#2954 Procedures - Add support for Oracle's PL/SQL "TYPE" matching in variable declerations Feature requests: BUG#17153 Cursors impossible on dynamic SQL Oracle, "Fundamentals of the PL/SQL Language" http://www.stanford.edu/dept/itss/docs/oracle/10g/appdev.101/b10807/02_funds.htm#i27306
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