parse_collate.c

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/*-------------------------------------------------------------------------
 *
 * parse_collate.c
 *      Routines for assigning collation information.
 *
 * We choose to handle collation analysis in a post-pass over the output
 * of expression parse analysis.  This is because we need more state to
 * perform this processing than is needed in the finished tree.  If we
 * did it on-the-fly while building the tree, all that state would have
 * to be kept in expression node trees permanently.  This way, the extra
 * storage is just local variables in this recursive routine.
 *
 * The info that is actually saved in the finished tree is:
 * 1. The output collation of each expression node, or InvalidOid if it
 * returns a noncollatable data type.  This can also be InvalidOid if the
 * result type is collatable but the collation is indeterminate.
 * 2. The collation to be used in executing each function.  InvalidOid means
 * that there are no collatable inputs or their collation is indeterminate.
 * This value is only stored in node types that might call collation-using
 * functions.
 *
 * You might think we could get away with storing only one collation per
 * node, but the two concepts really need to be kept distinct.  Otherwise
 * it's too confusing when a function produces a collatable output type but
 * has no collatable inputs or produces noncollatable output from collatable
 * inputs.
 *
 * Cases with indeterminate collation might result in an error being thrown
 * at runtime.  If we knew exactly which functions require collation
 * information, we could throw those errors at parse time instead.
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/parser/parse_collate.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "catalog/pg_aggregate.h"
#include "catalog/pg_collation.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
#include "parser/parse_collate.h"
#include "utils/lsyscache.h"


/*
 * Collation strength (the SQL standard calls this "derivation").  Order is
 * chosen to allow comparisons to work usefully.  Note: the standard doesn't
 * seem to distinguish between NONE and CONFLICT.
 */
typedef enum
{
    COLLATE_NONE,               /* expression is of a noncollatable datatype */
    COLLATE_IMPLICIT,           /* collation was derived implicitly */
    COLLATE_CONFLICT,           /* we had a conflict of implicit collations */
    COLLATE_EXPLICIT            /* collation was derived explicitly */
} CollateStrength;

typedef struct
{
    ParseState *pstate;         /* parse state (for error reporting) */
    Oid         collation;      /* OID of current collation, if any */
    CollateStrength strength;   /* strength of current collation choice */
    int         location;       /* location of expr that set collation */
    /* Remaining fields are only valid when strength == COLLATE_CONFLICT */
    Oid         collation2;     /* OID of conflicting collation */
    int         location2;      /* location of expr that set collation2 */
} assign_collations_context;

static bool assign_query_collations_walker(Node *node, ParseState *pstate);
static bool assign_collations_walker(Node *node,
                         assign_collations_context *context);
static void merge_collation_state(Oid collation,
                      CollateStrength strength,
                      int location,
                      Oid collation2,
                      int location2,
                      assign_collations_context *context);
static void assign_aggregate_collations(Aggref *aggref,
                            assign_collations_context *loccontext);
static void assign_ordered_set_collations(Aggref *aggref,
                              assign_collations_context *loccontext);
static void assign_hypothetical_collations(Aggref *aggref,
                               assign_collations_context *loccontext);


/*
 * assign_query_collations()
 *      Mark all expressions in the given Query with collation information.
 *
 * This should be applied to each Query after completion of parse analysis
 * for expressions.  Note that we do not recurse into sub-Queries, since
 * those should have been processed when built.
 */
void
assign_query_collations(ParseState *pstate, Query *query)
{
    /*
     * We just use query_tree_walker() to visit all the contained expressions.
     * We can skip the rangetable and CTE subqueries, though, since RTEs and
     * subqueries had better have been processed already (else Vars referring
     * to them would not get created with the right collation).
     */
    (void) query_tree_walker(query,
                             assign_query_collations_walker,
                             (void *) pstate,
                             QTW_IGNORE_RANGE_TABLE |
                             QTW_IGNORE_CTE_SUBQUERIES);
}

/*
 * Walker for assign_query_collations
 *
 * Each expression found by query_tree_walker is processed independently.
 * Note that query_tree_walker may pass us a whole List, such as the
 * targetlist, in which case each subexpression must be processed
 * independently --- we don't want to bleat if two different targetentries
 * have different collations.
 */
static bool
assign_query_collations_walker(Node *node, ParseState *pstate)
{
    /* Need do nothing for empty subexpressions */
    if (node == NULL)
        return false;

    /*
     * We don't want to recurse into a set-operations tree; it's already been
     * fully processed in transformSetOperationStmt.
     */
    if (IsA(node, SetOperationStmt))
        return false;

    if (IsA(node, List))
        assign_list_collations(pstate, (List *) node);
    else
        assign_expr_collations(pstate, node);

    return false;
}

/*
 * assign_list_collations()
 *      Mark all nodes in the list of expressions with collation information.
 *
 * The list member expressions are processed independently; they do not have
 * to share a common collation.
 */
void
assign_list_collations(ParseState *pstate, List *exprs)
{
    ListCell   *lc;

    foreach(lc, exprs)
    {
        Node       *node = (Node *) lfirst(lc);

        assign_expr_collations(pstate, node);
    }
}

/*
 * assign_expr_collations()
 *      Mark all nodes in the given expression tree with collation information.
 *
 * This is exported for the benefit of various utility commands that process
 * expressions without building a complete Query.  It should be applied after
 * calling transformExpr() plus any expression-modifying operations such as
 * coerce_to_boolean().
 */
void
assign_expr_collations(ParseState *pstate, Node *expr)
{
    assign_collations_context context;

    /* initialize context for tree walk */
    context.pstate = pstate;
    context.collation = InvalidOid;
    context.strength = COLLATE_NONE;
    context.location = -1;

    /* and away we go */
    (void) assign_collations_walker(expr, &context);
}

/*
 * select_common_collation()
 *      Identify a common collation for a list of expressions.
 *
 * The expressions should all return the same datatype, else this is not
 * terribly meaningful.
 *
 * none_ok means that it is permitted to return InvalidOid, indicating that
 * no common collation could be identified, even for collatable datatypes.
 * Otherwise, an error is thrown for conflict of implicit collations.
 *
 * In theory, none_ok = true reflects the rules of SQL standard clause "Result
 * of data type combinations", none_ok = false reflects the rules of clause
 * "Collation determination" (in some cases invoked via "Grouping
 * operations").
 */
Oid
select_common_collation(ParseState *pstate, List *exprs, bool none_ok)
{
    assign_collations_context context;

    /* initialize context for tree walk */
    context.pstate = pstate;
    context.collation = InvalidOid;
    context.strength = COLLATE_NONE;
    context.location = -1;

    /* and away we go */
    (void) assign_collations_walker((Node *) exprs, &context);

    /* deal with collation conflict */
    if (context.strength == COLLATE_CONFLICT)
    {
        if (none_ok)
            return InvalidOid;
        ereport(ERROR,
                (errcode(ERRCODE_COLLATION_MISMATCH),
                 errmsg("collation mismatch between implicit collations \"%s\" and \"%s\"",
                        get_collation_name(context.collation),
                        get_collation_name(context.collation2)),
                 errhint("You can choose the collation by applying the COLLATE clause to one or both expressions."),
                 parser_errposition(context.pstate, context.location2)));
    }

    /*
     * Note: if strength is still COLLATE_NONE, we'll return InvalidOid, but
     * that's okay because it must mean none of the expressions returned
     * collatable datatypes.
     */
    return context.collation;
}

/*
 * assign_collations_walker()
 *      Recursive guts of collation processing.
 *
 * Nodes with no children (eg, Vars, Consts, Params) must have been marked
 * when built.  All upper-level nodes are marked here.
 *
 * Note: if this is invoked directly on a List, it will attempt to infer a
 * common collation for all the list members.  In particular, it will throw
 * error if there are conflicting explicit collations for different members.
 */
static bool
assign_collations_walker(Node *node, assign_collations_context *context)
{
    assign_collations_context loccontext;
    Oid         collation;
    CollateStrength strength;
    int         location;

    /* Need do nothing for empty subexpressions */
    if (node == NULL)
        return false;

    /*
     * Prepare for recursion.  For most node types, though not all, the first
     * thing we do is recurse to process all nodes below this one. Each level
     * of the tree has its own local context.
     */
    loccontext.pstate = context->pstate;
    loccontext.collation = InvalidOid;
    loccontext.strength = COLLATE_NONE;
    loccontext.location = -1;
    /* Set these fields just to suppress uninitialized-value warnings: */
    loccontext.collation2 = InvalidOid;
    loccontext.location2 = -1;

    /*
     * Recurse if appropriate, then determine the collation for this node.
     *
     * Note: the general cases are at the bottom of the switch, after various
     * special cases.
     */
    switch (nodeTag(node))
    {
        case T_CollateExpr:
            {
                /*
                 * COLLATE sets an explicitly derived collation, regardless of
                 * what the child state is.  But we must recurse to set up
                 * collation info below here.
                 */
                CollateExpr *expr = (CollateExpr *) node;

                (void) expression_tree_walker(node,
                                              assign_collations_walker,
                                              (void *) &loccontext);

                collation = expr->collOid;
                Assert(OidIsValid(collation));
                strength = COLLATE_EXPLICIT;
                location = expr->location;
            }
            break;
        case T_FieldSelect:
            {
                /*
                 * For FieldSelect, the result has the field's declared
                 * collation, independently of what happened in the arguments.
                 * (The immediate argument must be composite and thus not
                 * collatable, anyhow.)  The field's collation was already
                 * looked up and saved in the node.
                 */
                FieldSelect *expr = (FieldSelect *) node;

                /* ... but first, recurse */
                (void) expression_tree_walker(node,
                                              assign_collations_walker,
                                              (void *) &loccontext);

                if (OidIsValid(expr->resultcollid))
                {
                    /* Node's result type is collatable. */
                    /* Pass up field's collation as an implicit choice. */
                    collation = expr->resultcollid;
                    strength = COLLATE_IMPLICIT;
                    location = exprLocation(node);
                }
                else
                {
                    /* Node's result type isn't collatable. */
                    collation = InvalidOid;
                    strength = COLLATE_NONE;
                    location = -1;      /* won't be used */
                }
            }
            break;
        case T_RowExpr:
            {
                /*
                 * RowExpr is a special case because the subexpressions are
                 * independent: we don't want to complain if some of them have
                 * incompatible explicit collations.
                 */
                RowExpr    *expr = (RowExpr *) node;

                assign_list_collations(context->pstate, expr->args);

                /*
                 * Since the result is always composite and therefore never
                 * has a collation, we can just stop here: this node has no
                 * impact on the collation of its parent.
                 */
                return false;   /* done */
            }
        case T_RowCompareExpr:
            {
                /*
                 * For RowCompare, we have to find the common collation of
                 * each pair of input columns and build a list.  If we can't
                 * find a common collation, we just put InvalidOid into the
                 * list, which may or may not cause an error at runtime.
                 */
                RowCompareExpr *expr = (RowCompareExpr *) node;
                List       *colls = NIL;
                ListCell   *l;
                ListCell   *r;

                forboth(l, expr->largs, r, expr->rargs)
                {
                    Node       *le = (Node *) lfirst(l);
                    Node       *re = (Node *) lfirst(r);
                    Oid         coll;

                    coll = select_common_collation(context->pstate,
                                                   list_make2(le, re),
                                                   true);
                    colls = lappend_oid(colls, coll);
                }
                expr->inputcollids = colls;

                /*
                 * Since the result is always boolean and therefore never has
                 * a collation, we can just stop here: this node has no impact
                 * on the collation of its parent.
                 */
                return false;   /* done */
            }
        case T_CoerceToDomain:
            {
                /*
                 * If the domain declaration included a non-default COLLATE
                 * spec, then use that collation as the output collation of
                 * the coercion.  Otherwise allow the input collation to
                 * bubble up.  (The input should be of the domain's base type,
                 * therefore we don't need to worry about it not being
                 * collatable when the domain is.)
                 */
                CoerceToDomain *expr = (CoerceToDomain *) node;
                Oid         typcollation = get_typcollation(expr->resulttype);

                /* ... but first, recurse */
                (void) expression_tree_walker(node,
                                              assign_collations_walker,
                                              (void *) &loccontext);

                if (OidIsValid(typcollation))
                {
                    /* Node's result type is collatable. */
                    if (typcollation == DEFAULT_COLLATION_OID)
                    {
                        /* Collation state bubbles up from child. */
                        collation = loccontext.collation;
                        strength = loccontext.strength;
                        location = loccontext.location;
                    }
                    else
                    {
                        /* Use domain's collation as an implicit choice. */
                        collation = typcollation;
                        strength = COLLATE_IMPLICIT;
                        location = exprLocation(node);
                    }
                }
                else
                {
                    /* Node's result type isn't collatable. */
                    collation = InvalidOid;
                    strength = COLLATE_NONE;
                    location = -1;      /* won't be used */
                }

                /*
                 * Save the state into the expression node.  We know it
                 * doesn't care about input collation.
                 */
                if (strength == COLLATE_CONFLICT)
                    exprSetCollation(node, InvalidOid);
                else
                    exprSetCollation(node, collation);
            }
            break;
        case T_TargetEntry:
            (void) expression_tree_walker(node,
                                          assign_collations_walker,
                                          (void *) &loccontext);

            /*
             * TargetEntry can have only one child, and should bubble that
             * state up to its parent.  We can't use the general-case code
             * below because exprType and friends don't work on TargetEntry.
             */
            collation = loccontext.collation;
            strength = loccontext.strength;
            location = loccontext.location;

            /*
             * Throw error if the collation is indeterminate for a TargetEntry
             * that is a sort/group target.  We prefer to do this now, instead
             * of leaving the comparison functions to fail at runtime, because
             * we can give a syntax error pointer to help locate the problem.
             * There are some cases where there might not be a failure, for
             * example if the planner chooses to use hash aggregation instead
             * of sorting for grouping; but it seems better to predictably
             * throw an error.  (Compare transformSetOperationTree, which will
             * throw error for indeterminate collation of set-op columns, even
             * though the planner might be able to implement the set-op
             * without sorting.)
             */
            if (strength == COLLATE_CONFLICT &&
                ((TargetEntry *) node)->ressortgroupref != 0)
                ereport(ERROR,
                        (errcode(ERRCODE_COLLATION_MISMATCH),
                         errmsg("collation mismatch between implicit collations \"%s\" and \"%s\"",
                                get_collation_name(loccontext.collation),
                                get_collation_name(loccontext.collation2)),
                         errhint("You can choose the collation by applying the COLLATE clause to one or both expressions."),
                         parser_errposition(context->pstate,
                                            loccontext.location2)));
            break;
        case T_InferenceElem:
        case T_RangeTblRef:
        case T_JoinExpr:
        case T_FromExpr:
        case T_OnConflictExpr:
        case T_SortGroupClause:
            (void) expression_tree_walker(node,
                                          assign_collations_walker,
                                          (void *) &loccontext);

            /*
             * When we're invoked on a query's jointree, we don't need to do
             * anything with join nodes except recurse through them to process
             * WHERE/ON expressions.  So just stop here.  Likewise, we don't
             * need to do anything when invoked on sort/group lists.
             */
            return false;
        case T_Query:
            {
                /*
                 * We get here when we're invoked on the Query belonging to a
                 * SubLink.  Act as though the Query returns its first output
                 * column, which indeed is what it does for EXPR_SUBLINK and
                 * ARRAY_SUBLINK cases.  In the cases where the SubLink
                 * returns boolean, this info will be ignored.  Special case:
                 * in EXISTS, the Query might return no columns, in which case
                 * we need do nothing.
                 *
                 * We needn't recurse, since the Query is already processed.
                 */
                Query      *qtree = (Query *) node;
                TargetEntry *tent;

                if (qtree->targetList == NIL)
                    return false;
                tent = (TargetEntry *) linitial(qtree->targetList);
                Assert(IsA(tent, TargetEntry));
                if (tent->resjunk)
                    return false;

                collation = exprCollation((Node *) tent->expr);
                /* collation doesn't change if it's converted to array */
                strength = COLLATE_IMPLICIT;
                location = exprLocation((Node *) tent->expr);
            }
            break;
        case T_List:
            (void) expression_tree_walker(node,
                                          assign_collations_walker,
                                          (void *) &loccontext);

            /*
             * When processing a list, collation state just bubbles up from
             * the list elements.
             */
            collation = loccontext.collation;
            strength = loccontext.strength;
            location = loccontext.location;
            break;

        case T_Var:
        case T_Const:
        case T_Param:
        case T_CoerceToDomainValue:
        case T_CaseTestExpr:
        case T_SetToDefault:
        case T_CurrentOfExpr:

            /*
             * General case for childless expression nodes.  These should
             * already have a collation assigned; it is not this function's
             * responsibility to look into the catalogs for base-case
             * information.
             */
            collation = exprCollation(node);

            /*
             * Note: in most cases, there will be an assigned collation
             * whenever type_is_collatable(exprType(node)); but an exception
             * occurs for a Var referencing a subquery output column for which
             * a unique collation was not determinable.  That may lead to a
             * runtime failure if a collation-sensitive function is applied to
             * the Var.
             */

            if (OidIsValid(collation))
                strength = COLLATE_IMPLICIT;
            else
                strength = COLLATE_NONE;
            location = exprLocation(node);
            break;

        default:
            {
                /*
                 * General case for most expression nodes with children. First
                 * recurse, then figure out what to assign to this node.
                 */
                Oid         typcollation;

                /*
                 * For most node types, we want to treat all the child
                 * expressions alike; but there are a few exceptions, hence
                 * this inner switch.
                 */
                switch (nodeTag(node))
                {
                    case T_Aggref:
                        {
                            /*
                             * Aggref is messy enough that we give it its own
                             * function, in fact three of them.  The FILTER
                             * clause is independent of the rest of the
                             * aggregate, however, so it can be processed
                             * separately.
                             */
                            Aggref     *aggref = (Aggref *) node;

                            switch (aggref->aggkind)
                            {
                                case AGGKIND_NORMAL:
                                    assign_aggregate_collations(aggref,
                                                                &loccontext);
                                    break;
                                case AGGKIND_ORDERED_SET:
                                    assign_ordered_set_collations(aggref,
                                                                &loccontext);
                                    break;
                                case AGGKIND_HYPOTHETICAL:
                                    assign_hypothetical_collations(aggref,
                                                                &loccontext);
                                    break;
                                default:
                                    elog(ERROR, "unrecognized aggkind: %d",
                                         (int) aggref->aggkind);
                            }

                            assign_expr_collations(context->pstate,
                                                 (Node *) aggref->aggfilter);
                        }
                        break;
                    case T_WindowFunc:
                        {
                            /*
                             * WindowFunc requires special processing only for
                             * its aggfilter clause, as for aggregates.
                             */
                            WindowFunc *wfunc = (WindowFunc *) node;

                            (void) assign_collations_walker((Node *) wfunc->args,
                                                            &loccontext);

                            assign_expr_collations(context->pstate,
                                                   (Node *) wfunc->aggfilter);
                        }
                        break;
                    case T_CaseExpr:
                        {
                            /*
                             * CaseExpr is a special case because we do not
                             * want to recurse into the test expression (if
                             * any).  It was already marked with collations
                             * during transformCaseExpr, and furthermore its
                             * collation is not relevant to the result of the
                             * CASE --- only the output expressions are.
                             */
                            CaseExpr   *expr = (CaseExpr *) node;
                            ListCell   *lc;

                            foreach(lc, expr->args)
                            {
                                CaseWhen   *when = (CaseWhen *) lfirst(lc);

                                Assert(IsA(when, CaseWhen));

                                /*
                                 * The condition expressions mustn't affect
                                 * the CASE's result collation either; but
                                 * since they are known to yield boolean, it's
                                 * safe to recurse directly on them --- they
                                 * won't change loccontext.
                                 */
                                (void) assign_collations_walker((Node *) when->expr,
                                                                &loccontext);
                                (void) assign_collations_walker((Node *) when->result,
                                                                &loccontext);
                            }
                            (void) assign_collations_walker((Node *) expr->defresult,
                                                            &loccontext);
                        }
                        break;
                    default:

                        /*
                         * Normal case: all child expressions contribute
                         * equally to loccontext.
                         */
                        (void) expression_tree_walker(node,
                                                    assign_collations_walker,
                                                      (void *) &loccontext);
                        break;
                }

                /*
                 * Now figure out what collation to assign to this node.
                 */
                typcollation = get_typcollation(exprType(node));
                if (OidIsValid(typcollation))
                {
                    /* Node's result is collatable; what about its input? */
                    if (loccontext.strength > COLLATE_NONE)
                    {
                        /* Collation state bubbles up from children. */
                        collation = loccontext.collation;
                        strength = loccontext.strength;
                        location = loccontext.location;
                    }
                    else
                    {
                        /*
                         * Collatable output produced without any collatable
                         * input.  Use the type's collation (which is usually
                         * DEFAULT_COLLATION_OID, but might be different for a
                         * domain).
                         */
                        collation = typcollation;
                        strength = COLLATE_IMPLICIT;
                        location = exprLocation(node);
                    }
                }
                else
                {
                    /* Node's result type isn't collatable. */
                    collation = InvalidOid;
                    strength = COLLATE_NONE;
                    location = -1;      /* won't be used */
                }

                /*
                 * Save the result collation into the expression node. If the
                 * state is COLLATE_CONFLICT, we'll set the collation to
                 * InvalidOid, which might result in an error at runtime.
                 */
                if (strength == COLLATE_CONFLICT)
                    exprSetCollation(node, InvalidOid);
                else
                    exprSetCollation(node, collation);

                /*
                 * Likewise save the input collation, which is the one that
                 * any function called by this node should use.
                 */
                if (loccontext.strength == COLLATE_CONFLICT)
                    exprSetInputCollation(node, InvalidOid);
                else
                    exprSetInputCollation(node, loccontext.collation);
            }
            break;
    }

    /*
     * Now, merge my information into my parent's state.
     */
    merge_collation_state(collation,
                          strength,
                          location,
                          loccontext.collation2,
                          loccontext.location2,
                          context);

    return false;
}

/*
 * Merge collation state of a subexpression into the context for its parent.
 */
static void
merge_collation_state(Oid collation,
                      CollateStrength strength,
                      int location,
                      Oid collation2,
                      int location2,
                      assign_collations_context *context)
{
    /*
     * If the collation strength for this node is different from what's
     * already in *context, then this node either dominates or is dominated by
     * earlier siblings.
     */
    if (strength > context->strength)
    {
        /* Override previous parent state */
        context->collation = collation;
        context->strength = strength;
        context->location = location;
        /* Bubble up error info if applicable */
        if (strength == COLLATE_CONFLICT)
        {
            context->collation2 = collation2;
            context->location2 = location2;
        }
    }
    else if (strength == context->strength)
    {
        /* Merge, or detect error if there's a collation conflict */
        switch (strength)
        {
            case COLLATE_NONE:
                /* Nothing + nothing is still nothing */
                break;
            case COLLATE_IMPLICIT:
                if (collation != context->collation)
                {
                    /*
                     * Non-default implicit collation always beats default.
                     */
                    if (context->collation == DEFAULT_COLLATION_OID)
                    {
                        /* Override previous parent state */
                        context->collation = collation;
                        context->strength = strength;
                        context->location = location;
                    }
                    else if (collation != DEFAULT_COLLATION_OID)
                    {
                        /*
                         * Ooops, we have a conflict.  We cannot throw error
                         * here, since the conflict could be resolved by a
                         * later sibling CollateExpr, or the parent might not
                         * care about collation anyway.  Return enough info to
                         * throw the error later, if needed.
                         */
                        context->strength = COLLATE_CONFLICT;
                        context->collation2 = collation;
                        context->location2 = location;
                    }
                }
                break;
            case COLLATE_CONFLICT:
                /* We're still conflicted ... */
                break;
            case COLLATE_EXPLICIT:
                if (collation != context->collation)
                {
                    /*
                     * Ooops, we have a conflict of explicit COLLATE clauses.
                     * Here we choose to throw error immediately; that is what
                     * the SQL standard says to do, and there's no good reason
                     * to be less strict.
                     */
                    ereport(ERROR,
                            (errcode(ERRCODE_COLLATION_MISMATCH),
                             errmsg("collation mismatch between explicit collations \"%s\" and \"%s\"",
                                    get_collation_name(context->collation),
                                    get_collation_name(collation)),
                             parser_errposition(context->pstate, location)));
                }
                break;
        }
    }
}

/*
 * Aggref is a special case because expressions used only for ordering
 * shouldn't be taken to conflict with each other or with regular args,
 * indeed shouldn't affect the aggregate's result collation at all.
 * We handle this by applying assign_expr_collations() to them rather than
 * passing down our loccontext.
 *
 * Note that we recurse to each TargetEntry, not directly to its contained
 * expression, so that the case above for T_TargetEntry will complain if we
 * can't resolve a collation for an ORDER BY item (whether or not it is also
 * a normal aggregate arg).
 *
 * We need not recurse into the aggorder or aggdistinct lists, because those
 * contain only SortGroupClause nodes which we need not process.
 */
static void
assign_aggregate_collations(Aggref *aggref,
                            assign_collations_context *loccontext)
{
    ListCell   *lc;

    /* Plain aggregates have no direct args */
    Assert(aggref->aggdirectargs == NIL);

    /* Process aggregated args, holding resjunk ones at arm's length */
    foreach(lc, aggref->args)
    {
        TargetEntry *tle = (TargetEntry *) lfirst(lc);

        Assert(IsA(tle, TargetEntry));
        if (tle->resjunk)
            assign_expr_collations(loccontext->pstate, (Node *) tle);
        else
            (void) assign_collations_walker((Node *) tle, loccontext);
    }
}

/*
 * For ordered-set aggregates, it's somewhat unclear how best to proceed.
 * The spec-defined inverse distribution functions have only one sort column
 * and don't return collatable types, but this is clearly too restrictive in
 * the general case.  Our solution is to consider that the aggregate's direct
 * arguments contribute normally to determination of the aggregate's own
 * collation, while aggregated arguments contribute only when the aggregate
 * is designed to have exactly one aggregated argument (i.e., it has a single
 * aggregated argument and is non-variadic).  If it can have more than one
 * aggregated argument, we process the aggregated arguments as independent
 * sort columns.  This avoids throwing error for something like
 *      agg(...) within group (order by x collate "foo", y collate "bar")
 * while also guaranteeing that variadic aggregates don't change in behavior
 * depending on how many sort columns a particular call happens to have.
 *
 * Otherwise this is much like the plain-aggregate case.
 */
static void
assign_ordered_set_collations(Aggref *aggref,
                              assign_collations_context *loccontext)
{
    bool        merge_sort_collations;
    ListCell   *lc;

    /* Merge sort collations to parent only if there can be only one */
    merge_sort_collations = (list_length(aggref->args) == 1 &&
                      get_func_variadictype(aggref->aggfnoid) == InvalidOid);

    /* Direct args, if any, are normal children of the Aggref node */
    (void) assign_collations_walker((Node *) aggref->aggdirectargs,
                                    loccontext);

    /* Process aggregated args appropriately */
    foreach(lc, aggref->args)
    {
        TargetEntry *tle = (TargetEntry *) lfirst(lc);

        Assert(IsA(tle, TargetEntry));
        if (merge_sort_collations)
            (void) assign_collations_walker((Node *) tle, loccontext);
        else
            assign_expr_collations(loccontext->pstate, (Node *) tle);
    }
}

/*
 * Hypothetical-set aggregates are even more special: per spec, we need to
 * unify the collations of each pair of hypothetical and aggregated args.
 * And we need to force the choice of collation down into the sort column
 * to ensure that the sort happens with the chosen collation.  Other than
 * that, the behavior is like regular ordered-set aggregates.  Note that
 * hypothetical direct arguments contribute to the aggregate collation
 * only when their partner aggregated arguments do.
 */
static void
assign_hypothetical_collations(Aggref *aggref,
                               assign_collations_context *loccontext)
{
    ListCell   *h_cell = list_head(aggref->aggdirectargs);
    ListCell   *s_cell = list_head(aggref->args);
    bool        merge_sort_collations;
    int         extra_args;

    /* Merge sort collations to parent only if there can be only one */
    merge_sort_collations = (list_length(aggref->args) == 1 &&
                      get_func_variadictype(aggref->aggfnoid) == InvalidOid);

    /* Process any non-hypothetical direct args */
    extra_args = list_length(aggref->aggdirectargs) - list_length(aggref->args);
    Assert(extra_args >= 0);
    while (extra_args-- > 0)
    {
        (void) assign_collations_walker((Node *) lfirst(h_cell), loccontext);
        h_cell = lnext(h_cell);
    }

    /* Scan hypothetical args and aggregated args in parallel */
    while (h_cell && s_cell)
    {
        Node       *h_arg = (Node *) lfirst(h_cell);
        TargetEntry *s_tle = (TargetEntry *) lfirst(s_cell);
        assign_collations_context paircontext;

        /*
         * Assign collations internally in this pair of expressions, then
         * choose a common collation for them.  This should match
         * select_common_collation(), but we can't use that function as-is
         * because we need access to the whole collation state so we can
         * bubble it up to the aggregate function's level.
         */
        paircontext.pstate = loccontext->pstate;
        paircontext.collation = InvalidOid;
        paircontext.strength = COLLATE_NONE;
        paircontext.location = -1;
        /* Set these fields just to suppress uninitialized-value warnings: */
        paircontext.collation2 = InvalidOid;
        paircontext.location2 = -1;

        (void) assign_collations_walker(h_arg, &paircontext);
        (void) assign_collations_walker((Node *) s_tle->expr, &paircontext);

        /* deal with collation conflict */
        if (paircontext.strength == COLLATE_CONFLICT)
            ereport(ERROR,
                    (errcode(ERRCODE_COLLATION_MISMATCH),
                     errmsg("collation mismatch between implicit collations \"%s\" and \"%s\"",
                            get_collation_name(paircontext.collation),
                            get_collation_name(paircontext.collation2)),
                     errhint("You can choose the collation by applying the COLLATE clause to one or both expressions."),
                     parser_errposition(paircontext.pstate,
                                        paircontext.location2)));

        /*
         * At this point paircontext.collation can be InvalidOid only if the
         * type is not collatable; no need to do anything in that case.  If we
         * do have to change the sort column's collation, do it by inserting a
         * RelabelType node into the sort column TLE.
         *
         * XXX This is pretty grotty for a couple of reasons:
         * assign_collations_walker isn't supposed to be changing the
         * expression structure like this, and a parse-time change of
         * collation ought to be signaled by a CollateExpr not a RelabelType
         * (the use of RelabelType for collation marking is supposed to be a
         * planner/executor thing only).  But we have no better alternative.
         * In particular, injecting a CollateExpr could result in the
         * expression being interpreted differently after dump/reload, since
         * we might be effectively promoting an implicit collation to
         * explicit.  This kluge is relying on ruleutils.c not printing a
         * COLLATE clause for a RelabelType, and probably on some other
         * fragile behaviors.
         */
        if (OidIsValid(paircontext.collation) &&
            paircontext.collation != exprCollation((Node *) s_tle->expr))
        {
            s_tle->expr = (Expr *)
                makeRelabelType(s_tle->expr,
                                exprType((Node *) s_tle->expr),
                                exprTypmod((Node *) s_tle->expr),
                                paircontext.collation,
                                COERCE_IMPLICIT_CAST);
        }

        /*
         * If appropriate, merge this column's collation state up to the
         * aggregate function.
         */
        if (merge_sort_collations)
            merge_collation_state(paircontext.collation,
                                  paircontext.strength,
                                  paircontext.location,
                                  paircontext.collation2,
                                  paircontext.location2,
                                  loccontext);

        h_cell = lnext(h_cell);
        s_cell = lnext(s_cell);
    }
    Assert(h_cell == NULL && s_cell == NULL);
}