pg_ndistinct.c

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/*-------------------------------------------------------------------------
 *
 * pg_ndistinct.c
 *      pg_ndistinct data type support.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/adt/pg_ndistinct.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "common/int.h"
#include "common/jsonapi.h"
#include "lib/stringinfo.h"
#include "mb/pg_wchar.h"
#include "nodes/miscnodes.h"
#include "statistics/extended_stats_internal.h"
#include "statistics/statistics_format.h"
#include "utils/builtins.h"
#include "utils/fmgrprotos.h"
 
/* Parsing state data */
typedef enum
{
    NDIST_EXPECT_START = 0,
    NDIST_EXPECT_ITEM,
    NDIST_EXPECT_KEY,
    NDIST_EXPECT_ATTNUM_LIST,
    NDIST_EXPECT_ATTNUM,
    NDIST_EXPECT_NDISTINCT,
    NDIST_EXPECT_COMPLETE,
} NDistinctSemanticState;
 
typedef struct
{
    const char *str;
    NDistinctSemanticState state;
 
    List       *distinct_items; /* Accumulated complete MVNDistinctItems */
    Node       *escontext;
 
    bool        found_attributes;   /* Item has "attributes" key */
    bool        found_ndistinct;    /* Item has "ndistinct" key */
    List       *attnum_list;    /* Accumulated attribute numbers */
    int32       ndistinct;
} NDistinctParseState;
 
/*
 * Invoked at the start of each MVNDistinctItem.
 *
 * The entire JSON document should be one array of MVNDistinctItem objects.
 * If we are anywhere else in the document, it is an error.
 */
static JsonParseErrorType
ndistinct_object_start(void *state)
{
    NDistinctParseState *parse = state;
 
    switch (parse->state)
    {
        case NDIST_EXPECT_ITEM:
            /* Now we expect to see attributes/ndistinct keys */
            parse->state = NDIST_EXPECT_KEY;
            return JSON_SUCCESS;
 
        case NDIST_EXPECT_START:
            /* pg_ndistinct must begin with a '[' */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Initial element must be an array."));
            break;
 
        case NDIST_EXPECT_KEY:
            /* In an object, expecting key */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Expected an object key."));
            break;
 
        case NDIST_EXPECT_ATTNUM_LIST:
            /* Just followed an "attributes" key */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Value of \"%s\" must be an array of attribute numbers.",
                              PG_NDISTINCT_KEY_ATTRIBUTES));
            break;
 
        case NDIST_EXPECT_ATTNUM:
            /* In an attribute number list, expect only scalar integers */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Attribute lists can only contain attribute numbers."));
            break;
 
        case NDIST_EXPECT_NDISTINCT:
            /* Just followed an "ndistinct" key */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Value of \"%s\" must be an integer.",
                              PG_NDISTINCT_KEY_NDISTINCT));
            break;
 
        default:
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Unexpected parse state: %d", (int) parse->state));
            break;
    }
 
    return JSON_SEM_ACTION_FAILED;
}
 
/*
 * Invoked at the end of an object.
 *
 * Check to ensure that it was a complete MVNDistinctItem
 */
static JsonParseErrorType
ndistinct_object_end(void *state)
{
    NDistinctParseState *parse = state;
 
    int         natts = 0;
 
    MVNDistinctItem *item;
 
    if (parse->state != NDIST_EXPECT_KEY)
    {
        errsave(parse->escontext,
                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                errdetail("Unexpected parse state: %d", (int) parse->state));
        return JSON_SEM_ACTION_FAILED;
    }
 
    if (!parse->found_attributes)
    {
        errsave(parse->escontext,
                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                errdetail("Item must contain \"%s\" key.",
                          PG_NDISTINCT_KEY_ATTRIBUTES));
        return JSON_SEM_ACTION_FAILED;
    }
 
    if (!parse->found_ndistinct)
    {
        errsave(parse->escontext,
                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                errdetail("Item must contain \"%s\" key.",
                          PG_NDISTINCT_KEY_NDISTINCT));
        return JSON_SEM_ACTION_FAILED;
    }
 
    /*
     * We need at least two attribute numbers for a ndistinct item, anything
     * less is malformed.
     */
    natts = list_length(parse->attnum_list);
    if ((natts < 2) || (natts > STATS_MAX_DIMENSIONS))
    {
        errsave(parse->escontext,
                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                errdetail("The \"%s\" key must contain an array of at least %d and no more than %d attributes.",
                          PG_NDISTINCT_KEY_ATTRIBUTES, 2, STATS_MAX_DIMENSIONS));
        return JSON_SEM_ACTION_FAILED;
    }
 
    /* Create the MVNDistinctItem */
    item = palloc(sizeof(MVNDistinctItem));
    item->nattributes = natts;
    item->attributes = palloc0(natts * sizeof(AttrNumber));
    item->ndistinct = (double) parse->ndistinct;
 
    for (int i = 0; i < natts; i++)
        item->attributes[i] = (AttrNumber) list_nth_int(parse->attnum_list, i);
 
    parse->distinct_items = lappend(parse->distinct_items, (void *) item);
 
    /* reset item state vars */
    list_free(parse->attnum_list);
    parse->attnum_list = NIL;
    parse->ndistinct = 0;
    parse->found_attributes = false;
    parse->found_ndistinct = false;
 
    /* Now we are looking for the next MVNDistinctItem */
    parse->state = NDIST_EXPECT_ITEM;
    return JSON_SUCCESS;
}
 
 
/*
 * Invoked at the start of an array.
 *
 * ndistinct input format has two types of arrays, the outer MVNDistinctItem
 * array and the attribute number array within each MVNDistinctItem.
 */
static JsonParseErrorType
ndistinct_array_start(void *state)
{
    NDistinctParseState *parse = state;
 
    switch (parse->state)
    {
        case NDIST_EXPECT_ATTNUM_LIST:
            parse->state = NDIST_EXPECT_ATTNUM;
            break;
 
        case NDIST_EXPECT_START:
            parse->state = NDIST_EXPECT_ITEM;
            break;
 
        default:
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Array found in unexpected place."));
            return JSON_SEM_ACTION_FAILED;
    }
 
    return JSON_SUCCESS;
}
 
 
/*
 * Invoked at the end of an array.
 *
 * Arrays can never be empty.
 */
static JsonParseErrorType
ndistinct_array_end(void *state)
{
    NDistinctParseState *parse = state;
 
    switch (parse->state)
    {
        case NDIST_EXPECT_ATTNUM:
            if (list_length(parse->attnum_list) > 0)
            {
                /*
                 * The attribute number list is complete, look for more
                 * MVNDistinctItem keys.
                 */
                parse->state = NDIST_EXPECT_KEY;
                return JSON_SUCCESS;
            }
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("The \"%s\" key must be a non-empty array.",
                              PG_NDISTINCT_KEY_ATTRIBUTES));
            break;
 
        case NDIST_EXPECT_ITEM:
            if (list_length(parse->distinct_items) > 0)
            {
                /* Item list is complete, we are done. */
                parse->state = NDIST_EXPECT_COMPLETE;
                return JSON_SUCCESS;
            }
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Item array cannot be empty."));
            break;
 
        default:
 
            /*
             * This can only happen if a case was missed in
             * ndistinct_array_start().
             */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Array found in unexpected place."));
            break;
    }
 
    return JSON_SEM_ACTION_FAILED;
}
 
/*
 * Invoked at the start of a key/value field.
 *
 * The valid keys for the MVNDistinctItem object are:
 *   - attributes
 *   - ndistinct
 */
static JsonParseErrorType
ndistinct_object_field_start(void *state, char *fname, bool isnull)
{
    NDistinctParseState *parse = state;
 
    if (strcmp(fname, PG_NDISTINCT_KEY_ATTRIBUTES) == 0)
    {
        if (parse->found_attributes)
        {
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Multiple \"%s\" keys are not allowed.",
                              PG_NDISTINCT_KEY_ATTRIBUTES));
            return JSON_SEM_ACTION_FAILED;
        }
        parse->found_attributes = true;
        parse->state = NDIST_EXPECT_ATTNUM_LIST;
        return JSON_SUCCESS;
    }
 
    if (strcmp(fname, PG_NDISTINCT_KEY_NDISTINCT) == 0)
    {
        if (parse->found_ndistinct)
        {
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Multiple \"%s\" keys are not allowed.",
                              PG_NDISTINCT_KEY_NDISTINCT));
            return JSON_SEM_ACTION_FAILED;
        }
        parse->found_ndistinct = true;
        parse->state = NDIST_EXPECT_NDISTINCT;
        return JSON_SUCCESS;
    }
 
    errsave(parse->escontext,
            errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
            errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
            errdetail("Only allowed keys are \"%s\" and \"%s\".",
                      PG_NDISTINCT_KEY_ATTRIBUTES,
                      PG_NDISTINCT_KEY_NDISTINCT));
    return JSON_SEM_ACTION_FAILED;
}
 
/*
 * Invoked at the start of an array element.
 *
 * The overall structure of the datatype is an array, but there are also
 * arrays as the value of every attributes key.
 */
static JsonParseErrorType
ndistinct_array_element_start(void *state, bool isnull)
{
    const NDistinctParseState *parse = state;
 
    switch (parse->state)
    {
        case NDIST_EXPECT_ATTNUM:
            if (!isnull)
                return JSON_SUCCESS;
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Attribute number array cannot be null."));
            break;
 
        case NDIST_EXPECT_ITEM:
            if (!isnull)
                return JSON_SUCCESS;
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Item list elements cannot be null."));
 
            break;
 
        default:
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Unexpected array element."));
            break;
    }
 
    return JSON_SEM_ACTION_FAILED;
}
 
/*
 * Test for valid subsequent attribute number.
 *
 * If the previous value is positive, then current value must either be
 * greater than the previous value, or negative.
 *
 * If the previous value is negative, then the value must be less than
 * the previous value.
 *
 * Duplicate values are obviously not allowed, but that is already covered
 * by the rules listed above.
 */
static bool
valid_subsequent_attnum(AttrNumber prev, AttrNumber cur)
{
    Assert(prev != 0);
 
    if (prev > 0)
        return ((cur > prev) || (cur < 0));
 
    return (cur < prev);
}
 
/*
 * Handle scalar events from the ndistinct input parser.
 *
 * Override integer parse error messages and replace them with errors
 * specific to the context.
 */
static JsonParseErrorType
ndistinct_scalar(void *state, char *token, JsonTokenType tokentype)
{
    NDistinctParseState *parse = state;
    AttrNumber  attnum;
    ErrorSaveContext escontext = {T_ErrorSaveContext};
 
    switch (parse->state)
    {
        case NDIST_EXPECT_ATTNUM:
            attnum = pg_strtoint16_safe(token, (Node *) &escontext);
 
            if (SOFT_ERROR_OCCURRED(&escontext))
            {
                errsave(parse->escontext,
                        errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                        errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                        errdetail("Invalid \"%s\" value.", PG_NDISTINCT_KEY_ATTRIBUTES));
                return JSON_SEM_ACTION_FAILED;
            }
 
            /*
             * The attribute number cannot be zero a negative number beyond
             * the number of the possible expressions.
             */
            if (attnum == 0 || attnum < (0 - STATS_MAX_DIMENSIONS))
            {
                errsave(parse->escontext,
                        errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                        errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                        errdetail("Invalid \"%s\" element: %d.",
                                  PG_NDISTINCT_KEY_ATTRIBUTES, attnum));
                return JSON_SEM_ACTION_FAILED;
            }
 
            if (list_length(parse->attnum_list) > 0)
            {
                const AttrNumber prev = llast_int(parse->attnum_list);
 
                if (!valid_subsequent_attnum(prev, attnum))
                {
                    errsave(parse->escontext,
                            errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                            errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                            errdetail("Invalid \"%s\" element: %d cannot follow %d.",
                                      PG_NDISTINCT_KEY_ATTRIBUTES, attnum, prev));
                    return JSON_SEM_ACTION_FAILED;
                }
            }
 
            parse->attnum_list = lappend_int(parse->attnum_list, (int) attnum);
            return JSON_SUCCESS;
 
        case NDIST_EXPECT_NDISTINCT:
 
            /*
             * While the structure dictates that ndistinct is a double
             * precision floating point, it has always been an integer in the
             * output generated.  Therefore, we parse it as an integer here.
             */
            parse->ndistinct = pg_strtoint32_safe(token, (Node *) &escontext);
 
            if (!SOFT_ERROR_OCCURRED(&escontext))
            {
                parse->state = NDIST_EXPECT_KEY;
                return JSON_SUCCESS;
            }
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Invalid \"%s\" value.",
                              PG_NDISTINCT_KEY_NDISTINCT));
            break;
 
        default:
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", parse->str),
                    errdetail("Unexpected scalar."));
            break;
    }
 
    return JSON_SEM_ACTION_FAILED;
}
 
/*
 * Compare the attribute arrays of two MVNDistinctItem values,
 * looking for duplicate sets. Return true if a duplicate set is found.
 *
 * The arrays are required to be in canonical order (all positive numbers
 * in ascending order first, followed by all negative numbers in descending
 * order) so it's safe to compare the attrnums in order, stopping at the
 * first difference.
 */
static bool
item_attributes_eq(const MVNDistinctItem *a, const MVNDistinctItem *b)
{
    if (a->nattributes != b->nattributes)
        return false;
 
    for (int i = 0; i < a->nattributes; i++)
    {
        if (a->attributes[i] != b->attributes[i])
            return false;
    }
 
    return true;
}
 
/*
 * Ensure that an attribute number appears as one of the attribute numbers
 * in a MVNDistinctItem.
 */
static bool
item_has_attnum(const MVNDistinctItem *item, AttrNumber attnum)
{
    for (int i = 0; i < item->nattributes; i++)
    {
        if (attnum == item->attributes[i])
            return true;
    }
    return false;
}
 
/*
 * Ensure that the attributes in MVNDistinctItem A are a subset of the
 * reference MVNDistinctItem B.
 */
static bool
item_is_attnum_subset(const MVNDistinctItem *item,
                      const MVNDistinctItem *refitem)
{
    for (int i = 0; i < item->nattributes; i++)
    {
        if (!item_has_attnum(refitem, item->attributes[i]))
            return false;
    }
    return true;
}
 
/*
 * Generate a string representing an array of attribute numbers.
 *
 * Freeing the allocated string is the responsibility of the caller.
 */
static char *
item_attnum_list(const MVNDistinctItem *item)
{
    StringInfoData str;
 
    initStringInfo(&str);
 
    appendStringInfo(&str, "%d", item->attributes[0]);
 
    for (int i = 1; i < item->nattributes; i++)
        appendStringInfo(&str, ", %d", item->attributes[i]);
 
    return str.data;
}
 
/*
 * Attempt to build and serialize the MVNDistinct object.
 *
 * This can only be executed after the completion of the JSON parsing.
 *
 * In the event of an error, set the error context and return NULL.
 */
static bytea *
build_mvndistinct(NDistinctParseState *parse, char *str)
{
    MVNDistinct *ndistinct;
    int         nitems = list_length(parse->distinct_items);
    bytea      *bytes;
    int         item_most_attrs = 0;
    int         item_most_attrs_idx = 0;
 
    switch (parse->state)
    {
        case NDIST_EXPECT_COMPLETE:
 
            /*
             * Parsing has ended correctly and we should have a list of items.
             * If we don't, something has been done wrong in one of the
             * earlier parsing steps.
             */
            if (nitems == 0)
                elog(ERROR,
                     "cannot have empty item list after parsing success.");
            break;
 
        case NDIST_EXPECT_START:
            /* blank */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", str),
                    errdetail("Value cannot be empty."));
            return NULL;
 
        default:
            /* Unexpected end-state. */
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", str),
                    errdetail("Unexpected end state %d.", parse->state));
            return NULL;
    }
 
    ndistinct = palloc(offsetof(MVNDistinct, items) +
                       nitems * sizeof(MVNDistinctItem));
 
    ndistinct->magic = STATS_NDISTINCT_MAGIC;
    ndistinct->type = STATS_NDISTINCT_TYPE_BASIC;
    ndistinct->nitems = nitems;
 
    for (int i = 0; i < nitems; i++)
    {
        MVNDistinctItem *item = list_nth(parse->distinct_items, i);
 
        /*
         * Ensure that this item does not duplicate the attributes of any
         * pre-existing item.
         */
        for (int j = 0; j < i; j++)
        {
            if (item_attributes_eq(item, &ndistinct->items[j]))
            {
                char       *s = item_attnum_list(item);
 
                errsave(parse->escontext,
                        errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                        errmsg("malformed pg_ndistinct: \"%s\"", str),
                        errdetail("Duplicated \"%s\" array found: [%s]",
                                  PG_NDISTINCT_KEY_ATTRIBUTES, s));
                pfree(s);
                return NULL;
            }
        }
 
        ndistinct->items[i].ndistinct = item->ndistinct;
        ndistinct->items[i].nattributes = item->nattributes;
 
        /*
         * This transfers free-ing responsibility from the distinct_items list
         * to the ndistinct object.
         */
        ndistinct->items[i].attributes = item->attributes;
 
        /*
         * Keep track of the first longest attribute list. All other attribute
         * lists must be a subset of this list.
         */
        if (item->nattributes > item_most_attrs)
        {
            item_most_attrs = item->nattributes;
            item_most_attrs_idx = i;
        }
    }
 
    /*
     * Verify that all the sets of attribute numbers are a proper subset of
     * the longest set recorded.  This acts as an extra sanity check based on
     * the input given.  Note that this still needs to be cross-checked with
     * the extended statistics objects this would be assigned to, but it
     * provides one extra layer of protection.
     */
    for (int i = 0; i < nitems; i++)
    {
        if (i == item_most_attrs_idx)
            continue;
 
        if (!item_is_attnum_subset(&ndistinct->items[i],
                                   &ndistinct->items[item_most_attrs_idx]))
        {
            const MVNDistinctItem *item = &ndistinct->items[i];
            const MVNDistinctItem *refitem = &ndistinct->items[item_most_attrs_idx];
            char       *item_list = item_attnum_list(item);
            char       *refitem_list = item_attnum_list(refitem);
 
            errsave(parse->escontext,
                    errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                    errmsg("malformed pg_ndistinct: \"%s\"", str),
                    errdetail("\"%s\" array: [%s] must be a subset of array: [%s]",
                              PG_NDISTINCT_KEY_ATTRIBUTES,
                              item_list, refitem_list));
            pfree(item_list);
            pfree(refitem_list);
            return NULL;
        }
    }
 
    bytes = statext_ndistinct_serialize(ndistinct);
 
    /*
     * Free the attribute lists, before the ndistinct itself.
     */
    for (int i = 0; i < nitems; i++)
        pfree(ndistinct->items[i].attributes);
    pfree(ndistinct);
 
    return bytes;
}
 
/*
 * pg_ndistinct_in
 *      input routine for type pg_ndistinct.
 */
Datum
pg_ndistinct_in(PG_FUNCTION_ARGS)
{
    char       *str = PG_GETARG_CSTRING(0);
    NDistinctParseState parse_state;
    JsonParseErrorType result;
    JsonLexContext *lex;
    JsonSemAction sem_action;
    bytea      *bytes = NULL;
 
    /* initialize semantic state */
    parse_state.str = str;
    parse_state.state = NDIST_EXPECT_START;
    parse_state.distinct_items = NIL;
    parse_state.escontext = fcinfo->context;
    parse_state.found_attributes = false;
    parse_state.found_ndistinct = false;
    parse_state.attnum_list = NIL;
    parse_state.ndistinct = 0;
 
    /* set callbacks */
    sem_action.semstate = (void *) &parse_state;
    sem_action.object_start = ndistinct_object_start;
    sem_action.object_end = ndistinct_object_end;
    sem_action.array_start = ndistinct_array_start;
    sem_action.array_end = ndistinct_array_end;
    sem_action.object_field_start = ndistinct_object_field_start;
    sem_action.object_field_end = NULL;
    sem_action.array_element_start = ndistinct_array_element_start;
    sem_action.array_element_end = NULL;
    sem_action.scalar = ndistinct_scalar;
 
    lex = makeJsonLexContextCstringLen(NULL, str, strlen(str),
                                       PG_UTF8, true);
    result = pg_parse_json(lex, &sem_action);
    freeJsonLexContext(lex);
 
    if (result == JSON_SUCCESS)
        bytes = build_mvndistinct(&parse_state, str);
 
    list_free(parse_state.attnum_list);
    list_free_deep(parse_state.distinct_items);
 
    if (bytes)
        PG_RETURN_BYTEA_P(bytes);
 
    /*
     * If escontext already set, just use that. Anything else is a generic
     * JSON parse error.
     */
    if (!SOFT_ERROR_OCCURRED(parse_state.escontext))
        errsave(parse_state.escontext,
                errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                errmsg("malformed pg_ndistinct: \"%s\"", str),
                errdetail("Must be valid JSON."));
 
    PG_RETURN_NULL();
}
 
/*
 * pg_ndistinct_out
 *      output routine for type pg_ndistinct
 *
 * Produces a human-readable representation of the value.
 */
Datum
pg_ndistinct_out(PG_FUNCTION_ARGS)
{
    bytea      *data = PG_GETARG_BYTEA_PP(0);
    MVNDistinct *ndist = statext_ndistinct_deserialize(data);
    int         i;
    StringInfoData str;
 
    initStringInfo(&str);
    appendStringInfoChar(&str, '[');
 
    for (i = 0; i < ndist->nitems; i++)
    {
        MVNDistinctItem item = ndist->items[i];
 
        if (i > 0)
            appendStringInfoString(&str, ", ");
 
        if (item.nattributes <= 0)
            elog(ERROR, "invalid zero-length attribute array in MVNDistinct");
 
        appendStringInfo(&str, "{\"" PG_NDISTINCT_KEY_ATTRIBUTES "\": [%d",
                         item.attributes[0]);
 
        for (int j = 1; j < item.nattributes; j++)
            appendStringInfo(&str, ", %d", item.attributes[j]);
 
        appendStringInfo(&str, "], \"" PG_NDISTINCT_KEY_NDISTINCT "\": %d}",
                         (int) item.ndistinct);
    }
 
    appendStringInfoChar(&str, ']');
 
    PG_RETURN_CSTRING(str.data);
}
 
/*
 * pg_ndistinct_recv
 *      binary input routine for type pg_ndistinct
 */
Datum
pg_ndistinct_recv(PG_FUNCTION_ARGS)
{
    ereport(ERROR,
            (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
             errmsg("cannot accept a value of type %s", "pg_ndistinct")));
 
    PG_RETURN_VOID();           /* keep compiler quiet */
}
 
/*
 * pg_ndistinct_send
 *      binary output routine for type pg_ndistinct
 *
 * n-distinct is serialized into a bytea value, so let's send that.
 */
Datum
pg_ndistinct_send(PG_FUNCTION_ARGS)
{
    return byteasend(fcinfo);
}