params.h

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/*-------------------------------------------------------------------------
 *
 * params.h
 *    Support for finding the values associated with Param nodes.
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/nodes/params.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef PARAMS_H
#define PARAMS_H

/* Forward declarations, to avoid including other headers */
struct Bitmapset;
struct ExprState;
struct Param;
struct ParseState;


/*
 *    ParamListInfo
 *
 *    ParamListInfo structures are used to pass parameters into the executor
 *    for parameterized plans.  We support two basic approaches to supplying
 *    parameter values, the "static" way and the "dynamic" way.
 *
 *    In the static approach, per-parameter data is stored in an array of
 *    ParamExternData structs appended to the ParamListInfo struct.
 *    Each entry in the array defines the value to be substituted for a
 *    PARAM_EXTERN parameter.  The "paramid" of a PARAM_EXTERN Param
 *    can range from 1 to numParams.
 *
 *    Although parameter numbers are normally consecutive, we allow
 *    ptype == InvalidOid to signal an unused array entry.
 *
 *    pflags is a flags field.  Currently the only used bit is:
 *    PARAM_FLAG_CONST signals the planner that it may treat this parameter
 *    as a constant (i.e., generate a plan that works only for this value
 *    of the parameter).
 *
 *    In the dynamic approach, all access to parameter values is done through
 *    hook functions found in the ParamListInfo struct.  In this case,
 *    the ParamExternData array is typically unused and not allocated;
 *    but the legal range of paramid is still 1 to numParams.
 *
 *    Although the data structure is really an array, not a list, we keep
 *    the old typedef name to avoid unnecessary code changes.
 *
 *    There are 3 hook functions that can be associated with a ParamListInfo
 *    structure:
 *
 *    If paramFetch isn't null, it is called to fetch the ParamExternData
 *    for a particular param ID, rather than accessing the relevant element
 *    of the ParamExternData array.  This supports the case where the array
 *    isn't there at all, as well as cases where the data in the array
 *    might be obsolete or lazily evaluated.  paramFetch must return the
 *    address of a ParamExternData struct describing the specified param ID;
 *    the convention above about ptype == InvalidOid signaling an invalid
 *    param ID still applies.  The returned struct can either be placed in
 *    the "workspace" supplied by the caller, or it can be in storage
 *    controlled by the paramFetch hook if that's more convenient.
 *    (In either case, the struct is not expected to be long-lived.)
 *    If "speculative" is true, the paramFetch hook should not risk errors
 *    in trying to fetch the parameter value, and should report an invalid
 *    parameter instead.
 *
 *    If paramCompile isn't null, then it controls what execExpr.c compiles
 *    for PARAM_EXTERN Param nodes --- typically, this hook would emit a
 *    EEOP_PARAM_CALLBACK step.  This allows unnecessary work to be
 *    optimized away in compiled expressions.
 *
 *    If parserSetup isn't null, then it is called to re-instantiate the
 *    original parsing hooks when a query needs to be re-parsed/planned.
 *    This is especially useful if the types of parameters might change
 *    from time to time, since it can replace the need to supply a fixed
 *    list of parameter types to the parser.
 *
 *    Notice that the paramFetch and paramCompile hooks are actually passed
 *    the ParamListInfo struct's address; they can therefore access all
 *    three of the "arg" fields, and the distinction between paramFetchArg
 *    and paramCompileArg is rather arbitrary.
 */

#define PARAM_FLAG_CONST    0x0001  /* parameter is constant */

typedef struct ParamExternData
{
    Datum       value;          /* parameter value */
    bool        isnull;         /* is it NULL? */
    uint16      pflags;         /* flag bits, see above */
    Oid         ptype;          /* parameter's datatype, or 0 */
} ParamExternData;

typedef struct ParamListInfoData *ParamListInfo;

typedef ParamExternData *(*ParamFetchHook) (ParamListInfo params,
                                            int paramid, bool speculative,
                                            ParamExternData *workspace);

typedef void (*ParamCompileHook) (ParamListInfo params, struct Param *param,
                                  struct ExprState *state,
                                  Datum *resv, bool *resnull);

typedef void (*ParserSetupHook) (struct ParseState *pstate, void *arg);

typedef struct ParamListInfoData
{
    ParamFetchHook paramFetch;  /* parameter fetch hook */
    void       *paramFetchArg;
    ParamCompileHook paramCompile;  /* parameter compile hook */
    void       *paramCompileArg;
    ParserSetupHook parserSetup;    /* parser setup hook */
    void       *parserSetupArg;
    int         numParams;      /* nominal/maximum # of Params represented */

    /*
     * params[] may be of length zero if paramFetch is supplied; otherwise it
     * must be of length numParams.
     */
    ParamExternData params[FLEXIBLE_ARRAY_MEMBER];
}           ParamListInfoData;


/* ----------------
 *    ParamExecData
 *
 *    ParamExecData entries are used for executor internal parameters
 *    (that is, values being passed into or out of a sub-query).  The
 *    paramid of a PARAM_EXEC Param is a (zero-based) index into an
 *    array of ParamExecData records, which is referenced through
 *    es_param_exec_vals or ecxt_param_exec_vals.
 *
 *    If execPlan is not NULL, it points to a SubPlanState node that needs
 *    to be executed to produce the value.  (This is done so that we can have
 *    lazy evaluation of InitPlans: they aren't executed until/unless a
 *    result value is needed.)  Otherwise the value is assumed to be valid
 *    when needed.
 * ----------------
 */

typedef struct ParamExecData
{
    void       *execPlan;       /* should be "SubPlanState *" */
    Datum       value;
    bool        isnull;
} ParamExecData;


/* Functions found in src/backend/nodes/params.c */
extern ParamListInfo makeParamList(int numParams);
extern ParamListInfo copyParamList(ParamListInfo from);
extern Size EstimateParamListSpace(ParamListInfo paramLI);
extern void SerializeParamList(ParamListInfo paramLI, char **start_address);
extern ParamListInfo RestoreParamList(char **start_address);

#endif                          /* PARAMS_H */