prepare.c

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/*-------------------------------------------------------------------------
 *
 * prepare.c
 *    Prepareable SQL statements via PREPARE, EXECUTE and DEALLOCATE
 *
 * This module also implements storage of prepared statements that are
 * accessed via the extended FE/BE query protocol.
 *
 *
 * Copyright (c) 2002-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/commands/prepare.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <limits.h>

#include "access/xact.h"
#include "catalog/pg_type.h"
#include "commands/createas.h"
#include "commands/prepare.h"
#include "miscadmin.h"
#include "nodes/nodeFuncs.h"
#include "parser/analyze.h"
#include "parser/parse_coerce.h"
#include "parser/parse_collate.h"
#include "parser/parse_expr.h"
#include "parser/parse_type.h"
#include "rewrite/rewriteHandler.h"
#include "tcop/pquery.h"
#include "tcop/utility.h"
#include "utils/builtins.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"


/*
 * The hash table in which prepared queries are stored. This is
 * per-backend: query plans are not shared between backends.
 * The keys for this hash table are the arguments to PREPARE and EXECUTE
 * (statement names); the entries are PreparedStatement structs.
 */
static HTAB *prepared_queries = NULL;

static void InitQueryHashTable(void);
static ParamListInfo EvaluateParams(ParseState *pstate,
                                    PreparedStatement *pstmt, List *params,
                                    EState *estate);
static Datum build_regtype_array(Oid *param_types, int num_params);

/*
 * Implements the 'PREPARE' utility statement.
 */
void
PrepareQuery(ParseState *pstate, PrepareStmt *stmt,
             int stmt_location, int stmt_len)
{
    RawStmt    *rawstmt;
    CachedPlanSource *plansource;
    Oid        *argtypes = NULL;
    int         nargs;
    Query      *query;
    List       *query_list;
    int         i;

    /*
     * Disallow empty-string statement name (conflicts with protocol-level
     * unnamed statement).
     */
    if (!stmt->name || stmt->name[0] == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PSTATEMENT_DEFINITION),
                 errmsg("invalid statement name: must not be empty")));

    /*
     * Need to wrap the contained statement in a RawStmt node to pass it to
     * parse analysis.
     *
     * Because parse analysis scribbles on the raw querytree, we must make a
     * copy to ensure we don't modify the passed-in tree.  FIXME someday.
     */
    rawstmt = makeNode(RawStmt);
    rawstmt->stmt = (Node *) copyObject(stmt->query);
    rawstmt->stmt_location = stmt_location;
    rawstmt->stmt_len = stmt_len;

    /*
     * Create the CachedPlanSource before we do parse analysis, since it needs
     * to see the unmodified raw parse tree.
     */
    plansource = CreateCachedPlan(rawstmt, pstate->p_sourcetext,
                                  CreateCommandTag(stmt->query));

    /* Transform list of TypeNames to array of type OIDs */
    nargs = list_length(stmt->argtypes);

    if (nargs)
    {
        ListCell   *l;

        argtypes = (Oid *) palloc(nargs * sizeof(Oid));
        i = 0;

        foreach(l, stmt->argtypes)
        {
            TypeName   *tn = lfirst(l);
            Oid         toid = typenameTypeId(pstate, tn);

            argtypes[i++] = toid;
        }
    }

    /*
     * Analyze the statement using these parameter types (any parameters
     * passed in from above us will not be visible to it), allowing
     * information about unknown parameters to be deduced from context.
     */
    query = parse_analyze_varparams(rawstmt, pstate->p_sourcetext,
                                    &argtypes, &nargs);

    /*
     * Check that all parameter types were determined.
     */
    for (i = 0; i < nargs; i++)
    {
        Oid         argtype = argtypes[i];

        if (argtype == InvalidOid || argtype == UNKNOWNOID)
            ereport(ERROR,
                    (errcode(ERRCODE_INDETERMINATE_DATATYPE),
                     errmsg("could not determine data type of parameter $%d",
                            i + 1)));
    }

    /*
     * grammar only allows PreparableStmt, so this check should be redundant
     */
    switch (query->commandType)
    {
        case CMD_SELECT:
        case CMD_INSERT:
        case CMD_UPDATE:
        case CMD_DELETE:
            /* OK */
            break;
        default:
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_PSTATEMENT_DEFINITION),
                     errmsg("utility statements cannot be prepared")));
            break;
    }

    /* Rewrite the query. The result could be 0, 1, or many queries. */
    query_list = QueryRewrite(query);

    /* Finish filling in the CachedPlanSource */
    CompleteCachedPlan(plansource,
                       query_list,
                       NULL,
                       argtypes,
                       nargs,
                       NULL,
                       NULL,
                       CURSOR_OPT_PARALLEL_OK,  /* allow parallel mode */
                       true);   /* fixed result */

    /*
     * Save the results.
     */
    StorePreparedStatement(stmt->name,
                           plansource,
                           true);
}

/*
 * ExecuteQuery --- implement the 'EXECUTE' utility statement.
 *
 * This code also supports CREATE TABLE ... AS EXECUTE.  That case is
 * indicated by passing a non-null intoClause.  The DestReceiver is already
 * set up correctly for CREATE TABLE AS, but we still have to make a few
 * other adjustments here.
 */
void
ExecuteQuery(ParseState *pstate,
             ExecuteStmt *stmt, IntoClause *intoClause,
             ParamListInfo params,
             DestReceiver *dest, QueryCompletion *qc)
{
    PreparedStatement *entry;
    CachedPlan *cplan;
    List       *plan_list;
    ParamListInfo paramLI = NULL;
    EState     *estate = NULL;
    Portal      portal;
    char       *query_string;
    int         eflags;
    long        count;

    /* Look it up in the hash table */
    entry = FetchPreparedStatement(stmt->name, true);

    /* Shouldn't find a non-fixed-result cached plan */
    if (!entry->plansource->fixed_result)
        elog(ERROR, "EXECUTE does not support variable-result cached plans");

    /* Evaluate parameters, if any */
    if (entry->plansource->num_params > 0)
    {
        /*
         * Need an EState to evaluate parameters; must not delete it till end
         * of query, in case parameters are pass-by-reference.  Note that the
         * passed-in "params" could possibly be referenced in the parameter
         * expressions.
         */
        estate = CreateExecutorState();
        estate->es_param_list_info = params;
        paramLI = EvaluateParams(pstate, entry, stmt->params, estate);
    }

    /* Create a new portal to run the query in */
    portal = CreateNewPortal();
    /* Don't display the portal in pg_cursors, it is for internal use only */
    portal->visible = false;

    /* Copy the plan's saved query string into the portal's memory */
    query_string = MemoryContextStrdup(portal->portalContext,
                                       entry->plansource->query_string);

    /* Replan if needed, and increment plan refcount for portal */
    cplan = GetCachedPlan(entry->plansource, paramLI, false, NULL);
    plan_list = cplan->stmt_list;

    /*
     * For CREATE TABLE ... AS EXECUTE, we must verify that the prepared
     * statement is one that produces tuples.  Currently we insist that it be
     * a plain old SELECT.  In future we might consider supporting other
     * things such as INSERT ... RETURNING, but there are a couple of issues
     * to be settled first, notably how WITH NO DATA should be handled in such
     * a case (do we really want to suppress execution?) and how to pass down
     * the OID-determining eflags (PortalStart won't handle them in such a
     * case, and for that matter it's not clear the executor will either).
     *
     * For CREATE TABLE ... AS EXECUTE, we also have to ensure that the proper
     * eflags and fetch count are passed to PortalStart/PortalRun.
     */
    if (intoClause)
    {
        PlannedStmt *pstmt;

        if (list_length(plan_list) != 1)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("prepared statement is not a SELECT")));
        pstmt = linitial_node(PlannedStmt, plan_list);
        if (pstmt->commandType != CMD_SELECT)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("prepared statement is not a SELECT")));

        /* Set appropriate eflags */
        eflags = GetIntoRelEFlags(intoClause);

        /* And tell PortalRun whether to run to completion or not */
        if (intoClause->skipData)
            count = 0;
        else
            count = FETCH_ALL;
    }
    else
    {
        /* Plain old EXECUTE */
        eflags = 0;
        count = FETCH_ALL;
    }

    PortalDefineQuery(portal,
                      NULL,
                      query_string,
                      entry->plansource->commandTag,
                      plan_list,
                      cplan);

    /*
     * Run the portal as appropriate.
     */
    PortalStart(portal, paramLI, eflags, GetActiveSnapshot());

    (void) PortalRun(portal, count, false, true, dest, dest, qc);

    PortalDrop(portal, false);

    if (estate)
        FreeExecutorState(estate);

    /* No need to pfree other memory, MemoryContext will be reset */
}

/*
 * EvaluateParams: evaluate a list of parameters.
 *
 * pstate: parse state
 * pstmt: statement we are getting parameters for.
 * params: list of given parameter expressions (raw parser output!)
 * estate: executor state to use.
 *
 * Returns a filled-in ParamListInfo -- this can later be passed to
 * CreateQueryDesc(), which allows the executor to make use of the parameters
 * during query execution.
 */
static ParamListInfo
EvaluateParams(ParseState *pstate, PreparedStatement *pstmt, List *params,
               EState *estate)
{
    Oid        *param_types = pstmt->plansource->param_types;
    int         num_params = pstmt->plansource->num_params;
    int         nparams = list_length(params);
    ParamListInfo paramLI;
    List       *exprstates;
    ListCell   *l;
    int         i;

    if (nparams != num_params)
        ereport(ERROR,
                (errcode(ERRCODE_SYNTAX_ERROR),
                 errmsg("wrong number of parameters for prepared statement \"%s\"",
                        pstmt->stmt_name),
                 errdetail("Expected %d parameters but got %d.",
                           num_params, nparams)));

    /* Quick exit if no parameters */
    if (num_params == 0)
        return NULL;

    /*
     * We have to run parse analysis for the expressions.  Since the parser is
     * not cool about scribbling on its input, copy first.
     */
    params = copyObject(params);

    i = 0;
    foreach(l, params)
    {
        Node       *expr = lfirst(l);
        Oid         expected_type_id = param_types[i];
        Oid         given_type_id;

        expr = transformExpr(pstate, expr, EXPR_KIND_EXECUTE_PARAMETER);

        given_type_id = exprType(expr);

        expr = coerce_to_target_type(pstate, expr, given_type_id,
                                     expected_type_id, -1,
                                     COERCION_ASSIGNMENT,
                                     COERCE_IMPLICIT_CAST,
                                     -1);

        if (expr == NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_DATATYPE_MISMATCH),
                     errmsg("parameter $%d of type %s cannot be coerced to the expected type %s",
                            i + 1,
                            format_type_be(given_type_id),
                            format_type_be(expected_type_id)),
                     errhint("You will need to rewrite or cast the expression."),
                     parser_errposition(pstate, exprLocation(lfirst(l)))));

        /* Take care of collations in the finished expression. */
        assign_expr_collations(pstate, expr);

        lfirst(l) = expr;
        i++;
    }

    /* Prepare the expressions for execution */
    exprstates = ExecPrepareExprList(params, estate);

    paramLI = makeParamList(num_params);

    i = 0;
    foreach(l, exprstates)
    {
        ExprState  *n = (ExprState *) lfirst(l);
        ParamExternData *prm = &paramLI->params[i];

        prm->ptype = param_types[i];
        prm->pflags = PARAM_FLAG_CONST;
        prm->value = ExecEvalExprSwitchContext(n,
                                               GetPerTupleExprContext(estate),
                                               &prm->isnull);

        i++;
    }

    return paramLI;
}


/*
 * Initialize query hash table upon first use.
 */
static void
InitQueryHashTable(void)
{
    HASHCTL     hash_ctl;

    hash_ctl.keysize = NAMEDATALEN;
    hash_ctl.entrysize = sizeof(PreparedStatement);

    prepared_queries = hash_create("Prepared Queries",
                                   32,
                                   &hash_ctl,
                                   HASH_ELEM | HASH_STRINGS);
}

/*
 * Store all the data pertaining to a query in the hash table using
 * the specified key.  The passed CachedPlanSource should be "unsaved"
 * in case we get an error here; we'll save it once we've created the hash
 * table entry.
 */
void
StorePreparedStatement(const char *stmt_name,
                       CachedPlanSource *plansource,
                       bool from_sql)
{
    PreparedStatement *entry;
    TimestampTz cur_ts = GetCurrentStatementStartTimestamp();
    bool        found;

    /* Initialize the hash table, if necessary */
    if (!prepared_queries)
        InitQueryHashTable();

    /* Add entry to hash table */
    entry = (PreparedStatement *) hash_search(prepared_queries,
                                              stmt_name,
                                              HASH_ENTER,
                                              &found);

    /* Shouldn't get a duplicate entry */
    if (found)
        ereport(ERROR,
                (errcode(ERRCODE_DUPLICATE_PSTATEMENT),
                 errmsg("prepared statement \"%s\" already exists",
                        stmt_name)));

    /* Fill in the hash table entry */
    entry->plansource = plansource;
    entry->from_sql = from_sql;
    entry->prepare_time = cur_ts;

    /* Now it's safe to move the CachedPlanSource to permanent memory */
    SaveCachedPlan(plansource);
}

/*
 * Lookup an existing query in the hash table. If the query does not
 * actually exist, throw ereport(ERROR) or return NULL per second parameter.
 *
 * Note: this does not force the referenced plancache entry to be valid,
 * since not all callers care.
 */
PreparedStatement *
FetchPreparedStatement(const char *stmt_name, bool throwError)
{
    PreparedStatement *entry;

    /*
     * If the hash table hasn't been initialized, it can't be storing
     * anything, therefore it couldn't possibly store our plan.
     */
    if (prepared_queries)
        entry = (PreparedStatement *) hash_search(prepared_queries,
                                                  stmt_name,
                                                  HASH_FIND,
                                                  NULL);
    else
        entry = NULL;

    if (!entry && throwError)
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_PSTATEMENT),
                 errmsg("prepared statement \"%s\" does not exist",
                        stmt_name)));

    return entry;
}

/*
 * Given a prepared statement, determine the result tupledesc it will
 * produce.  Returns NULL if the execution will not return tuples.
 *
 * Note: the result is created or copied into current memory context.
 */
TupleDesc
FetchPreparedStatementResultDesc(PreparedStatement *stmt)
{
    /*
     * Since we don't allow prepared statements' result tupdescs to change,
     * there's no need to worry about revalidating the cached plan here.
     */
    Assert(stmt->plansource->fixed_result);
    if (stmt->plansource->resultDesc)
        return CreateTupleDescCopy(stmt->plansource->resultDesc);
    else
        return NULL;
}

/*
 * Given a prepared statement that returns tuples, extract the query
 * targetlist.  Returns NIL if the statement doesn't have a determinable
 * targetlist.
 *
 * Note: this is pretty ugly, but since it's only used in corner cases like
 * Describe Statement on an EXECUTE command, we don't worry too much about
 * efficiency.
 */
List *
FetchPreparedStatementTargetList(PreparedStatement *stmt)
{
    List       *tlist;

    /* Get the plan's primary targetlist */
    tlist = CachedPlanGetTargetList(stmt->plansource, NULL);

    /* Copy into caller's context in case plan gets invalidated */
    return copyObject(tlist);
}

/*
 * Implements the 'DEALLOCATE' utility statement: deletes the
 * specified plan from storage.
 */
void
DeallocateQuery(DeallocateStmt *stmt)
{
    if (stmt->name)
        DropPreparedStatement(stmt->name, true);
    else
        DropAllPreparedStatements();
}

/*
 * Internal version of DEALLOCATE
 *
 * If showError is false, dropping a nonexistent statement is a no-op.
 */
void
DropPreparedStatement(const char *stmt_name, bool showError)
{
    PreparedStatement *entry;

    /* Find the query's hash table entry; raise error if wanted */
    entry = FetchPreparedStatement(stmt_name, showError);

    if (entry)
    {
        /* Release the plancache entry */
        DropCachedPlan(entry->plansource);

        /* Now we can remove the hash table entry */
        hash_search(prepared_queries, entry->stmt_name, HASH_REMOVE, NULL);
    }
}

/*
 * Drop all cached statements.
 */
void
DropAllPreparedStatements(void)
{
    HASH_SEQ_STATUS seq;
    PreparedStatement *entry;

    /* nothing cached */
    if (!prepared_queries)
        return;

    /* walk over cache */
    hash_seq_init(&seq, prepared_queries);
    while ((entry = hash_seq_search(&seq)) != NULL)
    {
        /* Release the plancache entry */
        DropCachedPlan(entry->plansource);

        /* Now we can remove the hash table entry */
        hash_search(prepared_queries, entry->stmt_name, HASH_REMOVE, NULL);
    }
}

/*
 * Implements the 'EXPLAIN EXECUTE' utility statement.
 *
 * "into" is NULL unless we are doing EXPLAIN CREATE TABLE AS EXECUTE,
 * in which case executing the query should result in creating that table.
 *
 * Note: the passed-in queryString is that of the EXPLAIN EXECUTE,
 * not the original PREPARE; we get the latter string from the plancache.
 */
void
ExplainExecuteQuery(ExecuteStmt *execstmt, IntoClause *into, ExplainState *es,
                    const char *queryString, ParamListInfo params,
                    QueryEnvironment *queryEnv)
{
    PreparedStatement *entry;
    const char *query_string;
    CachedPlan *cplan;
    List       *plan_list;
    ListCell   *p;
    ParamListInfo paramLI = NULL;
    EState     *estate = NULL;
    instr_time  planstart;
    instr_time  planduration;
    BufferUsage bufusage_start,
                bufusage;

    if (es->buffers)
        bufusage_start = pgBufferUsage;
    INSTR_TIME_SET_CURRENT(planstart);

    /* Look it up in the hash table */
    entry = FetchPreparedStatement(execstmt->name, true);

    /* Shouldn't find a non-fixed-result cached plan */
    if (!entry->plansource->fixed_result)
        elog(ERROR, "EXPLAIN EXECUTE does not support variable-result cached plans");

    query_string = entry->plansource->query_string;

    /* Evaluate parameters, if any */
    if (entry->plansource->num_params)
    {
        ParseState *pstate;

        pstate = make_parsestate(NULL);
        pstate->p_sourcetext = queryString;

        /*
         * Need an EState to evaluate parameters; must not delete it till end
         * of query, in case parameters are pass-by-reference.  Note that the
         * passed-in "params" could possibly be referenced in the parameter
         * expressions.
         */
        estate = CreateExecutorState();
        estate->es_param_list_info = params;

        paramLI = EvaluateParams(pstate, entry, execstmt->params, estate);
    }

    /* Replan if needed, and acquire a transient refcount */
    cplan = GetCachedPlan(entry->plansource, paramLI, true, queryEnv);

    INSTR_TIME_SET_CURRENT(planduration);
    INSTR_TIME_SUBTRACT(planduration, planstart);

    /* calc differences of buffer counters. */
    if (es->buffers)
    {
        memset(&bufusage, 0, sizeof(BufferUsage));
        BufferUsageAccumDiff(&bufusage, &pgBufferUsage, &bufusage_start);
    }

    plan_list = cplan->stmt_list;

    /* Explain each query */
    foreach(p, plan_list)
    {
        PlannedStmt *pstmt = lfirst_node(PlannedStmt, p);

        if (pstmt->commandType != CMD_UTILITY)
            ExplainOnePlan(pstmt, into, es, query_string, paramLI, queryEnv,
                           &planduration, (es->buffers ? &bufusage : NULL));
        else
            ExplainOneUtility(pstmt->utilityStmt, into, es, query_string,
                              paramLI, queryEnv);

        /* No need for CommandCounterIncrement, as ExplainOnePlan did it */

        /* Separate plans with an appropriate separator */
        if (lnext(plan_list, p) != NULL)
            ExplainSeparatePlans(es);
    }

    if (estate)
        FreeExecutorState(estate);

    ReleaseCachedPlan(cplan, true);
}

/*
 * This set returning function reads all the prepared statements and
 * returns a set of (name, statement, prepare_time, param_types, from_sql,
 * generic_plans, custom_plans).
 */
Datum
pg_prepared_statement(PG_FUNCTION_ARGS)
{
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    TupleDesc   tupdesc;
    Tuplestorestate *tupstore;
    MemoryContext per_query_ctx;
    MemoryContext oldcontext;

    /* check to see if caller supports us returning a tuplestore */
    if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("set-valued function called in context that cannot accept a set")));
    if (!(rsinfo->allowedModes & SFRM_Materialize))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("materialize mode required, but it is not allowed in this context")));

    /* need to build tuplestore in query context */
    per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
    oldcontext = MemoryContextSwitchTo(per_query_ctx);

    /*
     * build tupdesc for result tuples. This must match the definition of the
     * pg_prepared_statements view in system_views.sql
     */
    tupdesc = CreateTemplateTupleDesc(7);
    TupleDescInitEntry(tupdesc, (AttrNumber) 1, "name",
                       TEXTOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 2, "statement",
                       TEXTOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 3, "prepare_time",
                       TIMESTAMPTZOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 4, "parameter_types",
                       REGTYPEARRAYOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 5, "from_sql",
                       BOOLOID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 6, "generic_plans",
                       INT8OID, -1, 0);
    TupleDescInitEntry(tupdesc, (AttrNumber) 7, "custom_plans",
                       INT8OID, -1, 0);

    /*
     * We put all the tuples into a tuplestore in one scan of the hashtable.
     * This avoids any issue of the hashtable possibly changing between calls.
     */
    tupstore =
        tuplestore_begin_heap(rsinfo->allowedModes & SFRM_Materialize_Random,
                              false, work_mem);

    /* generate junk in short-term context */
    MemoryContextSwitchTo(oldcontext);

    /* hash table might be uninitialized */
    if (prepared_queries)
    {
        HASH_SEQ_STATUS hash_seq;
        PreparedStatement *prep_stmt;

        hash_seq_init(&hash_seq, prepared_queries);
        while ((prep_stmt = hash_seq_search(&hash_seq)) != NULL)
        {
            Datum       values[7];
            bool        nulls[7];

            MemSet(nulls, 0, sizeof(nulls));

            values[0] = CStringGetTextDatum(prep_stmt->stmt_name);
            values[1] = CStringGetTextDatum(prep_stmt->plansource->query_string);
            values[2] = TimestampTzGetDatum(prep_stmt->prepare_time);
            values[3] = build_regtype_array(prep_stmt->plansource->param_types,
                                            prep_stmt->plansource->num_params);
            values[4] = BoolGetDatum(prep_stmt->from_sql);
            values[5] = Int64GetDatumFast(prep_stmt->plansource->num_generic_plans);
            values[6] = Int64GetDatumFast(prep_stmt->plansource->num_custom_plans);

            tuplestore_putvalues(tupstore, tupdesc, values, nulls);
        }
    }

    /* clean up and return the tuplestore */
    tuplestore_donestoring(tupstore);

    rsinfo->returnMode = SFRM_Materialize;
    rsinfo->setResult = tupstore;
    rsinfo->setDesc = tupdesc;

    return (Datum) 0;
}

/*
 * This utility function takes a C array of Oids, and returns a Datum
 * pointing to a one-dimensional Postgres array of regtypes. An empty
 * array is returned as a zero-element array, not NULL.
 */
static Datum
build_regtype_array(Oid *param_types, int num_params)
{
    Datum      *tmp_ary;
    ArrayType  *result;
    int         i;

    tmp_ary = (Datum *) palloc(num_params * sizeof(Datum));

    for (i = 0; i < num_params; i++)
        tmp_ary[i] = ObjectIdGetDatum(param_types[i]);

    /* XXX: this hardcodes assumptions about the regtype type */
    result = construct_array(tmp_ary, num_params, REGTYPEOID,
                             4, true, TYPALIGN_INT);
    return PointerGetDatum(result);
}