parse_cte.c

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/*-------------------------------------------------------------------------
 *
 * parse_cte.c
 *    handle CTEs (common table expressions) in parser
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/parser/parse_cte.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "catalog/pg_collation.h"
#include "catalog/pg_type.h"
#include "nodes/nodeFuncs.h"
#include "parser/analyze.h"
#include "parser/parse_cte.h"
#include "utils/builtins.h"
#include "utils/lsyscache.h"


/* Enumeration of contexts in which a self-reference is disallowed */
typedef enum
{
    RECURSION_OK,
    RECURSION_NONRECURSIVETERM, /* inside the left-hand term */
    RECURSION_SUBLINK,          /* inside a sublink */
    RECURSION_OUTERJOIN,        /* inside nullable side of an outer join */
    RECURSION_INTERSECT,        /* underneath INTERSECT (ALL) */
    RECURSION_EXCEPT            /* underneath EXCEPT (ALL) */
} RecursionContext;

/* Associated error messages --- each must have one %s for CTE name */
static const char *const recursion_errormsgs[] = {
    /* RECURSION_OK */
    NULL,
    /* RECURSION_NONRECURSIVETERM */
    gettext_noop("recursive reference to query \"%s\" must not appear within its non-recursive term"),
    /* RECURSION_SUBLINK */
    gettext_noop("recursive reference to query \"%s\" must not appear within a subquery"),
    /* RECURSION_OUTERJOIN */
    gettext_noop("recursive reference to query \"%s\" must not appear within an outer join"),
    /* RECURSION_INTERSECT */
    gettext_noop("recursive reference to query \"%s\" must not appear within INTERSECT"),
    /* RECURSION_EXCEPT */
    gettext_noop("recursive reference to query \"%s\" must not appear within EXCEPT")
};

/*
 * For WITH RECURSIVE, we have to find an ordering of the clause members
 * with no forward references, and determine which members are recursive
 * (i.e., self-referential).  It is convenient to do this with an array
 * of CteItems instead of a list of CommonTableExprs.
 */
typedef struct CteItem
{
    CommonTableExpr *cte;       /* One CTE to examine */
    int         id;             /* Its ID number for dependencies */
    Bitmapset  *depends_on;     /* CTEs depended on (not including self) */
} CteItem;

/* CteState is what we need to pass around in the tree walkers */
typedef struct CteState
{
    /* global state: */
    ParseState *pstate;         /* global parse state */
    CteItem    *items;          /* array of CTEs and extra data */
    int         numitems;       /* number of CTEs */
    /* working state during a tree walk: */
    int         curitem;        /* index of item currently being examined */
    List       *innerwiths;     /* list of lists of CommonTableExpr */
    /* working state for checkWellFormedRecursion walk only: */
    int         selfrefcount;   /* number of self-references detected */
    RecursionContext context;   /* context to allow or disallow self-ref */
} CteState;


static void analyzeCTE(ParseState *pstate, CommonTableExpr *cte);

/* Dependency processing functions */
static void makeDependencyGraph(CteState *cstate);
static bool makeDependencyGraphWalker(Node *node, CteState *cstate);
static void TopologicalSort(ParseState *pstate, CteItem *items, int numitems);

/* Recursion validity checker functions */
static void checkWellFormedRecursion(CteState *cstate);
static bool checkWellFormedRecursionWalker(Node *node, CteState *cstate);
static void checkWellFormedSelectStmt(SelectStmt *stmt, CteState *cstate);


/*
 * transformWithClause -
 *    Transform the list of WITH clause "common table expressions" into
 *    Query nodes.
 *
 * The result is the list of transformed CTEs to be put into the output
 * Query.  (This is in fact the same as the ending value of p_ctenamespace,
 * but it seems cleaner to not expose that in the function's API.)
 */
List *
transformWithClause(ParseState *pstate, WithClause *withClause)
{
    ListCell   *lc;

    /* Only one WITH clause per query level */
    Assert(pstate->p_ctenamespace == NIL);
    Assert(pstate->p_future_ctes == NIL);

    /*
     * For either type of WITH, there must not be duplicate CTE names in the
     * list.  Check this right away so we needn't worry later.
     *
     * Also, tentatively mark each CTE as non-recursive, and initialize its
     * reference count to zero, and set pstate->p_hasModifyingCTE if needed.
     */
    foreach(lc, withClause->ctes)
    {
        CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);
        ListCell   *rest;

        for_each_cell(rest, withClause->ctes, lnext(withClause->ctes, lc))
        {
            CommonTableExpr *cte2 = (CommonTableExpr *) lfirst(rest);

            if (strcmp(cte->ctename, cte2->ctename) == 0)
                ereport(ERROR,
                        (errcode(ERRCODE_DUPLICATE_ALIAS),
                         errmsg("WITH query name \"%s\" specified more than once",
                                cte2->ctename),
                         parser_errposition(pstate, cte2->location)));
        }

        cte->cterecursive = false;
        cte->cterefcount = 0;

        if (!IsA(cte->ctequery, SelectStmt))
        {
            /* must be a data-modifying statement */
            Assert(IsA(cte->ctequery, InsertStmt) ||
                   IsA(cte->ctequery, UpdateStmt) ||
                   IsA(cte->ctequery, DeleteStmt));

            pstate->p_hasModifyingCTE = true;
        }
    }

    if (withClause->recursive)
    {
        /*
         * For WITH RECURSIVE, we rearrange the list elements if needed to
         * eliminate forward references.  First, build a work array and set up
         * the data structure needed by the tree walkers.
         */
        CteState    cstate;
        int         i;

        cstate.pstate = pstate;
        cstate.numitems = list_length(withClause->ctes);
        cstate.items = (CteItem *) palloc0(cstate.numitems * sizeof(CteItem));
        i = 0;
        foreach(lc, withClause->ctes)
        {
            cstate.items[i].cte = (CommonTableExpr *) lfirst(lc);
            cstate.items[i].id = i;
            i++;
        }

        /*
         * Find all the dependencies and sort the CteItems into a safe
         * processing order.  Also, mark CTEs that contain self-references.
         */
        makeDependencyGraph(&cstate);

        /*
         * Check that recursive queries are well-formed.
         */
        checkWellFormedRecursion(&cstate);

        /*
         * Set up the ctenamespace for parse analysis.  Per spec, all the WITH
         * items are visible to all others, so stuff them all in before parse
         * analysis.  We build the list in safe processing order so that the
         * planner can process the queries in sequence.
         */
        for (i = 0; i < cstate.numitems; i++)
        {
            CommonTableExpr *cte = cstate.items[i].cte;

            pstate->p_ctenamespace = lappend(pstate->p_ctenamespace, cte);
        }

        /*
         * Do parse analysis in the order determined by the topological sort.
         */
        for (i = 0; i < cstate.numitems; i++)
        {
            CommonTableExpr *cte = cstate.items[i].cte;

            analyzeCTE(pstate, cte);
        }
    }
    else
    {
        /*
         * For non-recursive WITH, just analyze each CTE in sequence and then
         * add it to the ctenamespace.  This corresponds to the spec's
         * definition of the scope of each WITH name.  However, to allow error
         * reports to be aware of the possibility of an erroneous reference,
         * we maintain a list in p_future_ctes of the not-yet-visible CTEs.
         */
        pstate->p_future_ctes = list_copy(withClause->ctes);

        foreach(lc, withClause->ctes)
        {
            CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);

            analyzeCTE(pstate, cte);
            pstate->p_ctenamespace = lappend(pstate->p_ctenamespace, cte);
            pstate->p_future_ctes = list_delete_first(pstate->p_future_ctes);
        }
    }

    return pstate->p_ctenamespace;
}


/*
 * Perform the actual parse analysis transformation of one CTE.  All
 * CTEs it depends on have already been loaded into pstate->p_ctenamespace,
 * and have been marked with the correct output column names/types.
 */
static void
analyzeCTE(ParseState *pstate, CommonTableExpr *cte)
{
    Query      *query;

    /* Analysis not done already */
    Assert(!IsA(cte->ctequery, Query));

    query = parse_sub_analyze(cte->ctequery, pstate, cte, false, true);
    cte->ctequery = (Node *) query;

    /*
     * Check that we got something reasonable.  These first two cases should
     * be prevented by the grammar.
     */
    if (!IsA(query, Query))
        elog(ERROR, "unexpected non-Query statement in WITH");
    if (query->utilityStmt != NULL)
        elog(ERROR, "unexpected utility statement in WITH");

    /*
     * We disallow data-modifying WITH except at the top level of a query,
     * because it's not clear when such a modification should be executed.
     */
    if (query->commandType != CMD_SELECT &&
        pstate->parentParseState != NULL)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("WITH clause containing a data-modifying statement must be at the top level"),
                 parser_errposition(pstate, cte->location)));

    /*
     * CTE queries are always marked not canSetTag.  (Currently this only
     * matters for data-modifying statements, for which the flag will be
     * propagated to the ModifyTable plan node.)
     */
    query->canSetTag = false;

    if (!cte->cterecursive)
    {
        /* Compute the output column names/types if not done yet */
        analyzeCTETargetList(pstate, cte, GetCTETargetList(cte));
    }
    else
    {
        /*
         * Verify that the previously determined output column types and
         * collations match what the query really produced.  We have to check
         * this because the recursive term could have overridden the
         * non-recursive term, and we don't have any easy way to fix that.
         */
        ListCell   *lctlist,
                   *lctyp,
                   *lctypmod,
                   *lccoll;
        int         varattno;

        lctyp = list_head(cte->ctecoltypes);
        lctypmod = list_head(cte->ctecoltypmods);
        lccoll = list_head(cte->ctecolcollations);
        varattno = 0;
        foreach(lctlist, GetCTETargetList(cte))
        {
            TargetEntry *te = (TargetEntry *) lfirst(lctlist);
            Node       *texpr;

            if (te->resjunk)
                continue;
            varattno++;
            Assert(varattno == te->resno);
            if (lctyp == NULL || lctypmod == NULL || lccoll == NULL)    /* shouldn't happen */
                elog(ERROR, "wrong number of output columns in WITH");
            texpr = (Node *) te->expr;
            if (exprType(texpr) != lfirst_oid(lctyp) ||
                exprTypmod(texpr) != lfirst_int(lctypmod))
                ereport(ERROR,
                        (errcode(ERRCODE_DATATYPE_MISMATCH),
                         errmsg("recursive query \"%s\" column %d has type %s in non-recursive term but type %s overall",
                                cte->ctename, varattno,
                                format_type_with_typemod(lfirst_oid(lctyp),
                                                         lfirst_int(lctypmod)),
                                format_type_with_typemod(exprType(texpr),
                                                         exprTypmod(texpr))),
                         errhint("Cast the output of the non-recursive term to the correct type."),
                         parser_errposition(pstate, exprLocation(texpr))));
            if (exprCollation(texpr) != lfirst_oid(lccoll))
                ereport(ERROR,
                        (errcode(ERRCODE_COLLATION_MISMATCH),
                         errmsg("recursive query \"%s\" column %d has collation \"%s\" in non-recursive term but collation \"%s\" overall",
                                cte->ctename, varattno,
                                get_collation_name(lfirst_oid(lccoll)),
                                get_collation_name(exprCollation(texpr))),
                         errhint("Use the COLLATE clause to set the collation of the non-recursive term."),
                         parser_errposition(pstate, exprLocation(texpr))));
            lctyp = lnext(cte->ctecoltypes, lctyp);
            lctypmod = lnext(cte->ctecoltypmods, lctypmod);
            lccoll = lnext(cte->ctecolcollations, lccoll);
        }
        if (lctyp != NULL || lctypmod != NULL || lccoll != NULL)    /* shouldn't happen */
            elog(ERROR, "wrong number of output columns in WITH");
    }
}

/*
 * Compute derived fields of a CTE, given the transformed output targetlist
 *
 * For a nonrecursive CTE, this is called after transforming the CTE's query.
 * For a recursive CTE, we call it after transforming the non-recursive term,
 * and pass the targetlist emitted by the non-recursive term only.
 *
 * Note: in the recursive case, the passed pstate is actually the one being
 * used to analyze the CTE's query, so it is one level lower down than in
 * the nonrecursive case.  This doesn't matter since we only use it for
 * error message context anyway.
 */
void
analyzeCTETargetList(ParseState *pstate, CommonTableExpr *cte, List *tlist)
{
    int         numaliases;
    int         varattno;
    ListCell   *tlistitem;

    /* Not done already ... */
    Assert(cte->ctecolnames == NIL);

    /*
     * We need to determine column names, types, and collations.  The alias
     * column names override anything coming from the query itself.  (Note:
     * the SQL spec says that the alias list must be empty or exactly as long
     * as the output column set; but we allow it to be shorter for consistency
     * with Alias handling.)
     */
    cte->ctecolnames = copyObject(cte->aliascolnames);
    cte->ctecoltypes = cte->ctecoltypmods = cte->ctecolcollations = NIL;
    numaliases = list_length(cte->aliascolnames);
    varattno = 0;
    foreach(tlistitem, tlist)
    {
        TargetEntry *te = (TargetEntry *) lfirst(tlistitem);
        Oid         coltype;
        int32       coltypmod;
        Oid         colcoll;

        if (te->resjunk)
            continue;
        varattno++;
        Assert(varattno == te->resno);
        if (varattno > numaliases)
        {
            char       *attrname;

            attrname = pstrdup(te->resname);
            cte->ctecolnames = lappend(cte->ctecolnames, makeString(attrname));
        }
        coltype = exprType((Node *) te->expr);
        coltypmod = exprTypmod((Node *) te->expr);
        colcoll = exprCollation((Node *) te->expr);

        /*
         * If the CTE is recursive, force the exposed column type of any
         * "unknown" column to "text".  We must deal with this here because
         * we're called on the non-recursive term before there's been any
         * attempt to force unknown output columns to some other type.  We
         * have to resolve unknowns before looking at the recursive term.
         *
         * The column might contain 'foo' COLLATE "bar", so don't override
         * collation if it's already set.
         */
        if (cte->cterecursive && coltype == UNKNOWNOID)
        {
            coltype = TEXTOID;
            coltypmod = -1;     /* should be -1 already, but be sure */
            if (!OidIsValid(colcoll))
                colcoll = DEFAULT_COLLATION_OID;
        }
        cte->ctecoltypes = lappend_oid(cte->ctecoltypes, coltype);
        cte->ctecoltypmods = lappend_int(cte->ctecoltypmods, coltypmod);
        cte->ctecolcollations = lappend_oid(cte->ctecolcollations, colcoll);
    }
    if (varattno < numaliases)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_COLUMN_REFERENCE),
                 errmsg("WITH query \"%s\" has %d columns available but %d columns specified",
                        cte->ctename, varattno, numaliases),
                 parser_errposition(pstate, cte->location)));
}


/*
 * Identify the cross-references of a list of WITH RECURSIVE items,
 * and sort into an order that has no forward references.
 */
static void
makeDependencyGraph(CteState *cstate)
{
    int         i;

    for (i = 0; i < cstate->numitems; i++)
    {
        CommonTableExpr *cte = cstate->items[i].cte;

        cstate->curitem = i;
        cstate->innerwiths = NIL;
        makeDependencyGraphWalker((Node *) cte->ctequery, cstate);
        Assert(cstate->innerwiths == NIL);
    }

    TopologicalSort(cstate->pstate, cstate->items, cstate->numitems);
}

/*
 * Tree walker function to detect cross-references and self-references of the
 * CTEs in a WITH RECURSIVE list.
 */
static bool
makeDependencyGraphWalker(Node *node, CteState *cstate)
{
    if (node == NULL)
        return false;
    if (IsA(node, RangeVar))
    {
        RangeVar   *rv = (RangeVar *) node;

        /* If unqualified name, might be a CTE reference */
        if (!rv->schemaname)
        {
            ListCell   *lc;
            int         i;

            /* ... but first see if it's captured by an inner WITH */
            foreach(lc, cstate->innerwiths)
            {
                List       *withlist = (List *) lfirst(lc);
                ListCell   *lc2;

                foreach(lc2, withlist)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc2);

                    if (strcmp(rv->relname, cte->ctename) == 0)
                        return false;   /* yes, so bail out */
                }
            }

            /* No, could be a reference to the query level we are working on */
            for (i = 0; i < cstate->numitems; i++)
            {
                CommonTableExpr *cte = cstate->items[i].cte;

                if (strcmp(rv->relname, cte->ctename) == 0)
                {
                    int         myindex = cstate->curitem;

                    if (i != myindex)
                    {
                        /* Add cross-item dependency */
                        cstate->items[myindex].depends_on =
                            bms_add_member(cstate->items[myindex].depends_on,
                                           cstate->items[i].id);
                    }
                    else
                    {
                        /* Found out this one is self-referential */
                        cte->cterecursive = true;
                    }
                    break;
                }
            }
        }
        return false;
    }
    if (IsA(node, SelectStmt))
    {
        SelectStmt *stmt = (SelectStmt *) node;
        ListCell   *lc;

        if (stmt->withClause)
        {
            if (stmt->withClause->recursive)
            {
                /*
                 * In the RECURSIVE case, all query names of the WITH are
                 * visible to all WITH items as well as the main query. So
                 * push them all on, process, pop them all off.
                 */
                cstate->innerwiths = lcons(stmt->withClause->ctes,
                                           cstate->innerwiths);
                foreach(lc, stmt->withClause->ctes)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);

                    (void) makeDependencyGraphWalker(cte->ctequery, cstate);
                }
                (void) raw_expression_tree_walker(node,
                                                  makeDependencyGraphWalker,
                                                  (void *) cstate);
                cstate->innerwiths = list_delete_first(cstate->innerwiths);
            }
            else
            {
                /*
                 * In the non-RECURSIVE case, query names are visible to the
                 * WITH items after them and to the main query.
                 */
                ListCell   *cell1;

                cstate->innerwiths = lcons(NIL, cstate->innerwiths);
                cell1 = list_head(cstate->innerwiths);
                foreach(lc, stmt->withClause->ctes)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);

                    (void) makeDependencyGraphWalker(cte->ctequery, cstate);
                    lfirst(cell1) = lappend((List *) lfirst(cell1), cte);
                }
                (void) raw_expression_tree_walker(node,
                                                  makeDependencyGraphWalker,
                                                  (void *) cstate);
                cstate->innerwiths = list_delete_first(cstate->innerwiths);
            }
            /* We're done examining the SelectStmt */
            return false;
        }
        /* if no WITH clause, just fall through for normal processing */
    }
    if (IsA(node, WithClause))
    {
        /*
         * Prevent raw_expression_tree_walker from recursing directly into a
         * WITH clause.  We need that to happen only under the control of the
         * code above.
         */
        return false;
    }
    return raw_expression_tree_walker(node,
                                      makeDependencyGraphWalker,
                                      (void *) cstate);
}

/*
 * Sort by dependencies, using a standard topological sort operation
 */
static void
TopologicalSort(ParseState *pstate, CteItem *items, int numitems)
{
    int         i,
                j;

    /* for each position in sequence ... */
    for (i = 0; i < numitems; i++)
    {
        /* ... scan the remaining items to find one that has no dependencies */
        for (j = i; j < numitems; j++)
        {
            if (bms_is_empty(items[j].depends_on))
                break;
        }

        /* if we didn't find one, the dependency graph has a cycle */
        if (j >= numitems)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("mutual recursion between WITH items is not implemented"),
                     parser_errposition(pstate, items[i].cte->location)));

        /*
         * Found one.  Move it to front and remove it from every other item's
         * dependencies.
         */
        if (i != j)
        {
            CteItem     tmp;

            tmp = items[i];
            items[i] = items[j];
            items[j] = tmp;
        }

        /*
         * Items up through i are known to have no dependencies left, so we
         * can skip them in this loop.
         */
        for (j = i + 1; j < numitems; j++)
        {
            items[j].depends_on = bms_del_member(items[j].depends_on,
                                                 items[i].id);
        }
    }
}


/*
 * Check that recursive queries are well-formed.
 */
static void
checkWellFormedRecursion(CteState *cstate)
{
    int         i;

    for (i = 0; i < cstate->numitems; i++)
    {
        CommonTableExpr *cte = cstate->items[i].cte;
        SelectStmt *stmt = (SelectStmt *) cte->ctequery;

        Assert(!IsA(stmt, Query));  /* not analyzed yet */

        /* Ignore items that weren't found to be recursive */
        if (!cte->cterecursive)
            continue;

        /* Must be a SELECT statement */
        if (!IsA(stmt, SelectStmt))
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_RECURSION),
                     errmsg("recursive query \"%s\" must not contain data-modifying statements",
                            cte->ctename),
                     parser_errposition(cstate->pstate, cte->location)));

        /* Must have top-level UNION */
        if (stmt->op != SETOP_UNION)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_RECURSION),
                     errmsg("recursive query \"%s\" does not have the form non-recursive-term UNION [ALL] recursive-term",
                            cte->ctename),
                     parser_errposition(cstate->pstate, cte->location)));

        /* The left-hand operand mustn't contain self-reference at all */
        cstate->curitem = i;
        cstate->innerwiths = NIL;
        cstate->selfrefcount = 0;
        cstate->context = RECURSION_NONRECURSIVETERM;
        checkWellFormedRecursionWalker((Node *) stmt->larg, cstate);
        Assert(cstate->innerwiths == NIL);

        /* Right-hand operand should contain one reference in a valid place */
        cstate->curitem = i;
        cstate->innerwiths = NIL;
        cstate->selfrefcount = 0;
        cstate->context = RECURSION_OK;
        checkWellFormedRecursionWalker((Node *) stmt->rarg, cstate);
        Assert(cstate->innerwiths == NIL);
        if (cstate->selfrefcount != 1)  /* shouldn't happen */
            elog(ERROR, "missing recursive reference");

        /* WITH mustn't contain self-reference, either */
        if (stmt->withClause)
        {
            cstate->curitem = i;
            cstate->innerwiths = NIL;
            cstate->selfrefcount = 0;
            cstate->context = RECURSION_SUBLINK;
            checkWellFormedRecursionWalker((Node *) stmt->withClause->ctes,
                                           cstate);
            Assert(cstate->innerwiths == NIL);
        }

        /*
         * Disallow ORDER BY and similar decoration atop the UNION. These
         * don't make sense because it's impossible to figure out what they
         * mean when we have only part of the recursive query's results. (If
         * we did allow them, we'd have to check for recursive references
         * inside these subtrees.)
         */
        if (stmt->sortClause)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("ORDER BY in a recursive query is not implemented"),
                     parser_errposition(cstate->pstate,
                                        exprLocation((Node *) stmt->sortClause))));
        if (stmt->limitOffset)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("OFFSET in a recursive query is not implemented"),
                     parser_errposition(cstate->pstate,
                                        exprLocation(stmt->limitOffset))));
        if (stmt->limitCount)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("LIMIT in a recursive query is not implemented"),
                     parser_errposition(cstate->pstate,
                                        exprLocation(stmt->limitCount))));
        if (stmt->lockingClause)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("FOR UPDATE/SHARE in a recursive query is not implemented"),
                     parser_errposition(cstate->pstate,
                                        exprLocation((Node *) stmt->lockingClause))));
    }
}

/*
 * Tree walker function to detect invalid self-references in a recursive query.
 */
static bool
checkWellFormedRecursionWalker(Node *node, CteState *cstate)
{
    RecursionContext save_context = cstate->context;

    if (node == NULL)
        return false;
    if (IsA(node, RangeVar))
    {
        RangeVar   *rv = (RangeVar *) node;

        /* If unqualified name, might be a CTE reference */
        if (!rv->schemaname)
        {
            ListCell   *lc;
            CommonTableExpr *mycte;

            /* ... but first see if it's captured by an inner WITH */
            foreach(lc, cstate->innerwiths)
            {
                List       *withlist = (List *) lfirst(lc);
                ListCell   *lc2;

                foreach(lc2, withlist)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc2);

                    if (strcmp(rv->relname, cte->ctename) == 0)
                        return false;   /* yes, so bail out */
                }
            }

            /* No, could be a reference to the query level we are working on */
            mycte = cstate->items[cstate->curitem].cte;
            if (strcmp(rv->relname, mycte->ctename) == 0)
            {
                /* Found a recursive reference to the active query */
                if (cstate->context != RECURSION_OK)
                    ereport(ERROR,
                            (errcode(ERRCODE_INVALID_RECURSION),
                             errmsg(recursion_errormsgs[cstate->context],
                                    mycte->ctename),
                             parser_errposition(cstate->pstate,
                                                rv->location)));
                /* Count references */
                if (++(cstate->selfrefcount) > 1)
                    ereport(ERROR,
                            (errcode(ERRCODE_INVALID_RECURSION),
                             errmsg("recursive reference to query \"%s\" must not appear more than once",
                                    mycte->ctename),
                             parser_errposition(cstate->pstate,
                                                rv->location)));
            }
        }
        return false;
    }
    if (IsA(node, SelectStmt))
    {
        SelectStmt *stmt = (SelectStmt *) node;
        ListCell   *lc;

        if (stmt->withClause)
        {
            if (stmt->withClause->recursive)
            {
                /*
                 * In the RECURSIVE case, all query names of the WITH are
                 * visible to all WITH items as well as the main query. So
                 * push them all on, process, pop them all off.
                 */
                cstate->innerwiths = lcons(stmt->withClause->ctes,
                                           cstate->innerwiths);
                foreach(lc, stmt->withClause->ctes)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);

                    (void) checkWellFormedRecursionWalker(cte->ctequery, cstate);
                }
                checkWellFormedSelectStmt(stmt, cstate);
                cstate->innerwiths = list_delete_first(cstate->innerwiths);
            }
            else
            {
                /*
                 * In the non-RECURSIVE case, query names are visible to the
                 * WITH items after them and to the main query.
                 */
                ListCell   *cell1;

                cstate->innerwiths = lcons(NIL, cstate->innerwiths);
                cell1 = list_head(cstate->innerwiths);
                foreach(lc, stmt->withClause->ctes)
                {
                    CommonTableExpr *cte = (CommonTableExpr *) lfirst(lc);

                    (void) checkWellFormedRecursionWalker(cte->ctequery, cstate);
                    lfirst(cell1) = lappend((List *) lfirst(cell1), cte);
                }
                checkWellFormedSelectStmt(stmt, cstate);
                cstate->innerwiths = list_delete_first(cstate->innerwiths);
            }
        }
        else
            checkWellFormedSelectStmt(stmt, cstate);
        /* We're done examining the SelectStmt */
        return false;
    }
    if (IsA(node, WithClause))
    {
        /*
         * Prevent raw_expression_tree_walker from recursing directly into a
         * WITH clause.  We need that to happen only under the control of the
         * code above.
         */
        return false;
    }
    if (IsA(node, JoinExpr))
    {
        JoinExpr   *j = (JoinExpr *) node;

        switch (j->jointype)
        {
            case JOIN_INNER:
                checkWellFormedRecursionWalker(j->larg, cstate);
                checkWellFormedRecursionWalker(j->rarg, cstate);
                checkWellFormedRecursionWalker(j->quals, cstate);
                break;
            case JOIN_LEFT:
                checkWellFormedRecursionWalker(j->larg, cstate);
                if (save_context == RECURSION_OK)
                    cstate->context = RECURSION_OUTERJOIN;
                checkWellFormedRecursionWalker(j->rarg, cstate);
                cstate->context = save_context;
                checkWellFormedRecursionWalker(j->quals, cstate);
                break;
            case JOIN_FULL:
                if (save_context == RECURSION_OK)
                    cstate->context = RECURSION_OUTERJOIN;
                checkWellFormedRecursionWalker(j->larg, cstate);
                checkWellFormedRecursionWalker(j->rarg, cstate);
                cstate->context = save_context;
                checkWellFormedRecursionWalker(j->quals, cstate);
                break;
            case JOIN_RIGHT:
                if (save_context == RECURSION_OK)
                    cstate->context = RECURSION_OUTERJOIN;
                checkWellFormedRecursionWalker(j->larg, cstate);
                cstate->context = save_context;
                checkWellFormedRecursionWalker(j->rarg, cstate);
                checkWellFormedRecursionWalker(j->quals, cstate);
                break;
            default:
                elog(ERROR, "unrecognized join type: %d",
                     (int) j->jointype);
        }
        return false;
    }
    if (IsA(node, SubLink))
    {
        SubLink    *sl = (SubLink *) node;

        /*
         * we intentionally override outer context, since subquery is
         * independent
         */
        cstate->context = RECURSION_SUBLINK;
        checkWellFormedRecursionWalker(sl->subselect, cstate);
        cstate->context = save_context;
        checkWellFormedRecursionWalker(sl->testexpr, cstate);
        return false;
    }
    return raw_expression_tree_walker(node,
                                      checkWellFormedRecursionWalker,
                                      (void *) cstate);
}

/*
 * subroutine for checkWellFormedRecursionWalker: process a SelectStmt
 * without worrying about its WITH clause
 */
static void
checkWellFormedSelectStmt(SelectStmt *stmt, CteState *cstate)
{
    RecursionContext save_context = cstate->context;

    if (save_context != RECURSION_OK)
    {
        /* just recurse without changing state */
        raw_expression_tree_walker((Node *) stmt,
                                   checkWellFormedRecursionWalker,
                                   (void *) cstate);
    }
    else
    {
        switch (stmt->op)
        {
            case SETOP_NONE:
            case SETOP_UNION:
                raw_expression_tree_walker((Node *) stmt,
                                           checkWellFormedRecursionWalker,
                                           (void *) cstate);
                break;
            case SETOP_INTERSECT:
                if (stmt->all)
                    cstate->context = RECURSION_INTERSECT;
                checkWellFormedRecursionWalker((Node *) stmt->larg,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->rarg,
                                               cstate);
                cstate->context = save_context;
                checkWellFormedRecursionWalker((Node *) stmt->sortClause,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->limitOffset,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->limitCount,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->lockingClause,
                                               cstate);
                /* stmt->withClause is intentionally ignored here */
                break;
            case SETOP_EXCEPT:
                if (stmt->all)
                    cstate->context = RECURSION_EXCEPT;
                checkWellFormedRecursionWalker((Node *) stmt->larg,
                                               cstate);
                cstate->context = RECURSION_EXCEPT;
                checkWellFormedRecursionWalker((Node *) stmt->rarg,
                                               cstate);
                cstate->context = save_context;
                checkWellFormedRecursionWalker((Node *) stmt->sortClause,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->limitOffset,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->limitCount,
                                               cstate);
                checkWellFormedRecursionWalker((Node *) stmt->lockingClause,
                                               cstate);
                /* stmt->withClause is intentionally ignored here */
                break;
            default:
                elog(ERROR, "unrecognized set op: %d",
                     (int) stmt->op);
        }
    }
}