path: root/Docs/sp-imp-spec.txt

	      Implementation specification for Stored Procedures
	      ==================================================

This is a first draft, only covering the basics for parsing, creating, and
calling a PROCEDURE.


- How parsing and execution of queries work

  In order to execute a query, the function sql_parse.cc:mysql_parse() is
  called, which in turn calls the parser (yyparse()) with an updated Lex
  structure as the result. mysql_parse() then calls mysql_execute_command()
  which dispatches on the command code (in Lex) to the corresponding code for
  executing that particular query.

  There are thre structures involved in the execution of a query which are of
  interest to the stored procedure implementation:

  - Lex (mentioned above) is the "compiled" query, that is the output from
    the parser and what is then interpreted to do the actual work.
    It constains an enum value (sql_command) which is the query type, and
    all the data collected by the parser needed for the execution  (table
    names, fields, values, etc).
  - THD is the "run-time" state of a connection, containing all that is
    needed for a particular client connection, and, among other things, the
    Lex structure currently being executed.
  - Item_*:  During parsing, all data is translated into "items", objects of
    the subclasses of "Item", such as Item_int, Item_real, Item_string, etc,
    for basic datatypes, and also various more specialized Item types for
    expressions to be evaluated (Item_func objects).


- How to fit Stored Procedure into this scheme

  - An overview of the classes and files for stored procedures
    (More detailed APIs at the end of this file)

    - class sp_head (sp_head.{cc,h})
      This contains, among other things, an array of "instructions" and the
      method for executing the procedure.

    - class sp_pcontext (sp_pcontext.{cc,h}
      This is the parse context for the procedure. It's primarily used during
      parsing to keep track of local parameters, variables and labels, but
      it's also used at CALL time do find parameters mode (IN, OUT or INOUT)
      and type when setting up the runtime context.

    - class sp_instr (sp_head.{cc,h})
      This is the base class for "instructions", that is, what is generated
      by the parser. It turns out that we only need 4 different sub classes:
      - sp_instr_stmt
        Execute a statement. This is the "call-out" any normal SQL statement,
        like a SELECT, INSERT etc. It contains the Lex structure for the
        statement in question.
      - sp_instr_set
        Set the value of a local variable (or parameter)
      - sp_instr_jump
        An unconditional jump.
      - sp_instr_jump_if_not
        Jump if condition is not true. It turns out that the negative test is
        most convenient when generating the code for the flow control
        constructs.

    - class sp_rcontext (sp_rcontext.h)
      This is the runtime context in the THD structure.
      It contains an array of items, the parameters and local variables for
      the currently executing stored procedure.
      This means that variable value lookup is in runtime is constant time,
      a simple index operation.

    - class Item_splocal (Item.{cc,h})
      This is a subclass of Item. Its sole purpose is to hide the fact that
      the real Item is actually in the current frame (runtime context).
      It contains the frame offset and defers all methods to the real Item
      in the frame. This is what the parser generates for local variables.

    - Utility functions (sp.{cc,h})
      This contains functions for creating, dropping and finding a stored
      procedure in the mysql.proc table (or internal cache, when it is
      implemented).


  - Parsing CREATE PROCEDURE ...

    When parsing a CREATE PROCEDURE the parser first initializes the
    sphead and spcont (runtime context) fields in the Lex.
    The sql_command code for the result of parsing a is
    SQLCOM_CREATE_PROCEDURE.

    The parsing of the parameter list and body is relatively
    straight-forward:

    - Parameters:
        name, type and mode (IN/OUT/INOUT) is pushed to spcont
    - Declared local variables:
        Same as parameters (mode is then IN)
    - Local Variable references:
        If an identifier is found in in spcont, an Item_splocal is created
        with the variable's frame index, otherwise an Item_field or Item_ref
        is created (as before).
    - Statements:
        The Lex in THD is replaced by a new Lex structure and the statement,
        is parsed as usual. A sp_instr_stmt is created, containing the new
        Lex, and added to added to the instructions in  sphead.
        Afterwards, the procedure's Lex is restored in THD.
    - SET var:
        Setting a local variable generates a sp_instr_set instruction,
        containing the variable's frame offset, the expression (an Item),
        and the type.
    - Flow control:
        Flow control constructs like, IF, WHILE, etc, generate a conditional
        and unconditional jumps in the "obvious" way, but a few notes may
        be required:
        - Forward jumps: When jumping forward, the exact destination is not
          known at the time of the creation of the jump instruction. The
          sphead therefore contains list of instruction-label pairs for
          each forward reference. When the position later is known, the
          instructions in the list are updated with the correct location.
        - Loop constructs have optional labels. If a loop doesn't have a
          label, an anonymous label is generated to simplify the parsing.
        - There are two types of CASE. The "simple" case is implemented
          with an anonymous variable bound to the value to be tested.


    - An example

      Parsing the procedure:

      create procedure a(s char(16))
      begin
        declare x int;
        set x = 3;
        while x > 0 do
          set x = x-1;
          insert into db.tab values (x, s);
        end while
      end

      would generate the following structures:
            ______
      thd: |      |     _________
           | lex -+--->|         |                     ___________________
           |______|    | spcont -+------------------->| "s",in,char(16):0 |
                       | sphead -+------              |("x",in,int     :1)|
                       |_________|      |             |___________________|
                                    ____V__________________
                                   | m_name: "a"           |
                                   | m_defstr: "create ..."|
                                   | m_instr: ...          |
                                   |_______________________|

      Note that the contents of the spcont is changing during the parsing,
      at all times reflecting the state of the would-be runtime frame.
      The m_instr is an array of instructions:

      Pos.  Instruction
       0    sp_instr_set(1, '3')
       1    sp_instr_jump_if_not(5, 'x>0')
       2    sp_instr_set(1, 'x-1')
       3    sp_instr_stmt('insert into ...')
       4    sp_instr_jump(1)
       5    <end>

     Here, '3', 'x>0', etc, represent the Items or Lex for the respective
     expressions or statements.


  - Parsing CREATE FUNCTION ...

    Creating a functions is essensially the same thing as for a PROCEDURE,
    with the addition that a FUNCTION has a return type and a RETURN
    statement, but no OUT or INOUT parameters.

    [QQ - More details here; sp_head needs a result slot and a type flag
          indicating if it's a function or procedure]


  - Storing, caching, dropping...

    As seen above, the entired definition string, including the "CREATE
    PROCEDURE" (or "FUNCTION") is kept. The procedure definition string is
    stored in the table mysql.proc with the name and type as the key, the
    type being one of the enum ("procedure","function").

    A PROCEDURE is just stored int the mysql.proc table. A FUNCTION has an
    additional requirement. They will be called in expressions with the same
    syntax as UDFs, so UDFs and stored FUNCTIONs share the namespace. Thus,
    we must make sure that we do not have UDFs and FUNCTIONs with the same
    name (even if they are storded in different places).

    This means that we can reparse the procedure as many time as we want.
    The first time, the resulting Lex is used to store the procedure in
    the database (using the function sp.c:sp_create_procedure()).

    The simplest way would be to just leave it at that, and re-read the
    procedure from the database each time it is called. (And in fact, that's
    the way the earliest implementation will work.)
    However, this is not very efficient, and we can do better. The full
    implementation should work like this:

    1) Upon creation time, parse and store the procedure. Note that we still
       need to parse it to catch syntax errors, but we can't check if called
       procedures exists for instance.
    2) Upon first CALL, read from the database, parse it, and cache the
       resulting Lex in memory. This time we can do more error checking.
    3) Upon subsequent CALLs, use the cached Lex.

    Note that this implies that the Lex structure with its sphead must be
    reentrant, that is, reusable and shareable between different threads
    and calls. The runtime state for a procedure is kept in the sp_rcontext
    in THD.

    The mechanisms of storing, finding, and dropping procedures are
    encapsulated in the files sp.{cc,h}.


  - CALLing a procedure

    A CALL is parsed just like any statement. The resulting Lex has the
    sql_command SQLCOM_CALL, the procedure's name and the parameters are
    pushed to the Lex' value_list.

    sql_parse.cc:mysql_execute_command() then uses sp.cc:sp_find() to
    get the sp_head for the procedure (which may have been read from the
    database or feetched from the in-memory cache) and calls the sp_head's
    method execute().
    Note: It's important that substatements called by the procedure do not
          do send_ok(). Fortunately, there is a flag in THD->net to disable
          this during CALLs. If a substatement fails, it will however send
          an error back to the client, so the CALL mechanism must return
          immediately and without sending an error.

    The sp_head::execute() method works as follows:

    1) Keep a pointer to the old runtime context in THD (if any)
    2) Create a new runtime context. The information about the required size
       is in sp_head's parse time context.
    3) Push each parameter (from the CALL's Lex->value_list) to the new
       context. If it's an OUT or INOUT parameter, the parameter's offset
       in the caller's frame is set in the new context as well.
    4) For each instruction, call its execute() method.
       The result is a pointer to the next instruction to execute (or NULL)
       if an error occured.
    5) On success, set the new values of the OUT and INOUT parameters in
       the caller's frame.


    - Evaluating Items

      There are three occasions where we need to evaluate an expression:

      - When SETing a variable
      - When CALLing a procedure
      - When testing an expression for a branch (in IF, WHILE, etc)

      The semantics in stored procedures is "call-by-value", so we have to
      evaluate any "func" Items at the point of the CALL or SET, otherwise
      we would get a kind of "lazy" evaluation with unexpected results with
      respect to OUT parameters for instance.
      For this the support function, sp_head.cc:eval_func_item() is needed.


  - Calling a FUNCTION

    Functions don't have an explicit call keyword like procedures. Instead,
    they appear in expressions with the conventional syntax "fun(arg, ...)".
    The problem is that we already have User Defined Functions (UDFs) which
    are called the same way. A UDF is detected by the lexical analyzer (not
    the parser!), in the find_keyword() function, and returns a UDF_*_FUNC
    or UDA_*_SUM token with the udf_func object as the yylval.

    So, stored functions must be handled in a simpilar way, and as a
    consequence, UDFs and functions must not have the same name.

    [QQ - Details of how function calls works here]


  - Parsing DROP PROCEDURE/FUNCTION

    The procedure name is pushed to Lex->value_list.
    The sql_command code for the result of parsing a is
    SQLCOM_DROP_PROCEDURE/SQLCOM_DROP_FUNCTION.

    Dropping is done by simply getting the procedure with the sp_find()
    function and calling sp_drop() (both in sp.{cc,h}).


  - Class and function APIs

    - The parser context: sp_pcontext.h

      typedef enum
      {
        sp_param_in,
	sp_param_out,
	sp_param_inout
      } sp_param_mode_t;

      typedef struct
      {
        Item_string *name;
	enum enum_field_types type;
	sp_param_mode_t mode;
	uint offset;			// Offset in current frame
	my_bool isset;
      } sp_pvar_t;

      class sp_pcontext
      {
        sp_pcontext();

	// Return the maximum frame size
	uint max_framesize();

	// Return the current frame size
	uint current_framesize();

	// Return the number of parameters
	uint params();

	// Set the number of parameters to the current frame size
	void set_params();

	// Set type of the variable at offset 'i' in the frame
	void set_type(uint i, enum enum_field_types type);

	// Mark the i:th variable to "set" (i.e. having a value) with
	// 'val' true.
	void set_isset(uint i, my_bool val);

	// Push the variable 'name' to the frame.
        void push(LEX_STRING *name,
	          enum enum_field_types type, sp_param_mode_t mode);

        // Pop 'num' variables from the frame.
        void pop(uint num = 1);

	// Find variable by name
	sp_pvar_t *find_pvar(LEX_STRING *name);

	// Find variable by index
	sp_pvar_t *find_pvar(uint i);

	// Push label 'name' of instruction index 'ip' to the label context
	sp_label_t *push_label(char *name, uint ip);

	// Find label 'name' in the context
	sp_label_t *find_label(char *name);

	// Return the last pushed label
	sp_label_t *last_label();

	// Return and remove the last pushed label.
	sp_label_t *pop_label();
      }


    - The run-time context (call frame): sp_rcontext.h

    class sp_rcontext
    {
      // 'size' is the max size of the context
      sp_rcontext(uint size);

      // Push value (parameter) 'i' to the frame
      void push_item(Item *i);

      // Set slot 'idx' to value 'i'
      void set_item(uint idx, Item *i);

      // Return the item in slot 'idx'
      Item *get_item(uint idx);

      // Set the "out" index 'oidx' for slot 'idx. If it's an IN slot,
      // use 'oidx' -1.
      void set_oindex(uint idx, int oidx);

      // Return the "out" index for slot 'idx'
      int get_oindex(uint idx);
    }


    - The procedure: sp_head.h

      class sp_head
      {
        sp_head(LEX_STRING *name, LEX*);

        // Store this procedure in the database. This is a wrapper around
        // the function sp_create_procedure().
        int create(THD *);

        // CALL this procedure.
        int execute(THD *);

        // Add the instruction to this procedure.
        void add_instr(sp_instr *);

        // Return the number of instructions.
        uint instructions();

        // Resets lex in 'thd' and keeps a copy of the old one.
        void reset_lex(THD *);

        // Restores lex in 'thd' from our copy, but keeps some status from the
        // one in 'thd', like ptr, tables, fields, etc.
        void restore_lex(THD *);

        // Put the instruction on the backpatch list, associated with
        // the label.
        void push_backpatch(sp_instr *, struct sp_label *);

        // Update all instruction with this label in the backpatch list to
        // the current position.
        void backpatch(struct sp_label *);
      }

    - Instructions

      - The base class:
        class sp_instr
        {
          // 'ip' is the index of this instruction
          sp_instr(uint ip);

          // Execute this instrution.
          // '*nextp' will be set to the index of the next instruction
          // to execute. (For most instruction this will be the
          // instruction following this one.)
          // Returns 0 on success, non-zero if some error occured.
          virtual int execute(THD *, uint *nextp)
        }

      - Statement instruction:
        class sp_instr_stmt : public sp_instr
        {
          sp_instr_stmt(uint ip);

          int execute(THD *, uint *nextp);

	  // Set the statement's Lex
          void set_lex(LEX *);

	  // Return the statement's Lex
	  LEX *get_lex();
        }

     -  SET instruction:
        class sp_instr_set : public sp_instr
	{
	  // 'offset' is the variable's frame offset, 'val' the value,
	  // and 'type' the variable type.
	  sp_instr_set(uint ip,
	               uint offset, Item *val, enum enum_field_types type);

	  int execute(THD *, uint *nextp);
	}

      - Unconditional jump
        class sp_instr_jump : public sp_instr
	{
	  // No destination, must be set.
	  sp_instr_jump(uint ip);

	  // 'dest' is the destination instruction index.
	  sp_instr_jump(uint ip, uint dest);

	  virtual int execute(THD *, uint *nextp);

	  // Set the destination instruction 'dest'.
	  void set_destination(uint dest);
        }

      - Conditional jump
        class sp_instr_jump_if_not : public sp_instr_jump
	{
	  // Jump if 'i' evaluates to false. Destination not set yet.
	  sp_instr_jump_if_not(uint ip, Item *i);

	  // Jump to 'dest' if 'i' evaluates to false.
	  sp_instr_jump_if_not(uint ip, Item *i, uint dest)

	  int execute(THD *, uint *nextp);
        }

    - Utility functions: sp.h

      // Finds a stored procedure given its name. Returns NULL if not found.
      sp_head *sp_find(THD *, Item_string *name);

      // Store the procedure 'name' in the database. 'def' is the complete
      // definition string ("create procedure ...").
      int sp_create_procedure(THD *,
                              char *name, uint namelen,
			      char *def, uint deflen);

      // Drop the procedure 'name' from the database.
      int sp_drop(THD *, char *name, uint namelen);

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