execGrouping.c

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/*-------------------------------------------------------------------------
 *
 * execGrouping.c
 *    executor utility routines for grouping, hashing, and aggregation
 *
 * Note: we currently assume that equality and hashing functions are not
 * collation-sensitive, so the code in this file has no support for passing
 * collation settings through from callers.  That may have to change someday.
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/execGrouping.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/parallel.h"
#include "executor/executor.h"
#include "miscadmin.h"
#include "utils/hashutils.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"

static uint32 TupleHashTableHash(struct tuplehash_hash *tb, const MinimalTuple tuple);
static int  TupleHashTableMatch(struct tuplehash_hash *tb, const MinimalTuple tuple1, const MinimalTuple tuple2);

/*
 * Define parameters for tuple hash table code generation. The interface is
 * *also* declared in execnodes.h (to generate the types, which are externally
 * visible).
 */
#define SH_PREFIX tuplehash
#define SH_ELEMENT_TYPE TupleHashEntryData
#define SH_KEY_TYPE MinimalTuple
#define SH_KEY firstTuple
#define SH_HASH_KEY(tb, key) TupleHashTableHash(tb, key)
#define SH_EQUAL(tb, a, b) TupleHashTableMatch(tb, a, b) == 0
#define SH_SCOPE extern
#define SH_STORE_HASH
#define SH_GET_HASH(tb, a) a->hash
#define SH_DEFINE
#include "lib/simplehash.h"


/*****************************************************************************
 *      Utility routines for grouping tuples together
 *****************************************************************************/

/*
 * execTuplesMatchPrepare
 *      Build expression that can be evaluated using ExecQual(), returning
 *      whether an ExprContext's inner/outer tuples are NOT DISTINCT
 */
ExprState *
execTuplesMatchPrepare(TupleDesc desc,
                       int numCols,
                       const AttrNumber *keyColIdx,
                       const Oid *eqOperators,
                       const Oid *collations,
                       PlanState *parent)
{
    Oid        *eqFunctions = (Oid *) palloc(numCols * sizeof(Oid));
    int         i;
    ExprState  *expr;

    if (numCols == 0)
        return NULL;

    /* lookup equality functions */
    for (i = 0; i < numCols; i++)
        eqFunctions[i] = get_opcode(eqOperators[i]);

    /* build actual expression */
    expr = ExecBuildGroupingEqual(desc, desc, NULL, NULL,
                                  numCols, keyColIdx, eqFunctions, collations,
                                  parent);

    return expr;
}

/*
 * execTuplesHashPrepare
 *      Look up the equality and hashing functions needed for a TupleHashTable.
 *
 * This is similar to execTuplesMatchPrepare, but we also need to find the
 * hash functions associated with the equality operators.  *eqFunctions and
 * *hashFunctions receive the palloc'd result arrays.
 *
 * Note: we expect that the given operators are not cross-type comparisons.
 */
void
execTuplesHashPrepare(int numCols,
                      const Oid *eqOperators,
                      Oid **eqFuncOids,
                      FmgrInfo **hashFunctions)
{
    int         i;

    *eqFuncOids = (Oid *) palloc(numCols * sizeof(Oid));
    *hashFunctions = (FmgrInfo *) palloc(numCols * sizeof(FmgrInfo));

    for (i = 0; i < numCols; i++)
    {
        Oid         eq_opr = eqOperators[i];
        Oid         eq_function;
        Oid         left_hash_function;
        Oid         right_hash_function;

        eq_function = get_opcode(eq_opr);
        if (!get_op_hash_functions(eq_opr,
                                   &left_hash_function, &right_hash_function))
            elog(ERROR, "could not find hash function for hash operator %u",
                 eq_opr);
        /* We're not supporting cross-type cases here */
        Assert(left_hash_function == right_hash_function);
        (*eqFuncOids)[i] = eq_function;
        fmgr_info(right_hash_function, &(*hashFunctions)[i]);
    }
}


/*****************************************************************************
 *      Utility routines for all-in-memory hash tables
 *
 * These routines build hash tables for grouping tuples together (eg, for
 * hash aggregation).  There is one entry for each not-distinct set of tuples
 * presented.
 *****************************************************************************/

/*
 * Construct an empty TupleHashTable
 *
 *  numCols, keyColIdx: identify the tuple fields to use as lookup key
 *  eqfunctions: equality comparison functions to use
 *  hashfunctions: datatype-specific hashing functions to use
 *  nbuckets: initial estimate of hashtable size
 *  additionalsize: size of data stored in ->additional
 *  metacxt: memory context for long-lived allocation, but not per-entry data
 *  tablecxt: memory context in which to store table entries
 *  tempcxt: short-lived context for evaluation hash and comparison functions
 *
 * The function arrays may be made with execTuplesHashPrepare().  Note they
 * are not cross-type functions, but expect to see the table datatype(s)
 * on both sides.
 *
 * Note that keyColIdx, eqfunctions, and hashfunctions must be allocated in
 * storage that will live as long as the hashtable does.
 */
TupleHashTable
BuildTupleHashTableExt(PlanState *parent,
                       TupleDesc inputDesc,
                       int numCols, AttrNumber *keyColIdx,
                       const Oid *eqfuncoids,
                       FmgrInfo *hashfunctions,
                       Oid *collations,
                       long nbuckets, Size additionalsize,
                       MemoryContext metacxt,
                       MemoryContext tablecxt,
                       MemoryContext tempcxt,
                       bool use_variable_hash_iv)
{
    TupleHashTable hashtable;
    Size        entrysize = sizeof(TupleHashEntryData) + additionalsize;
    MemoryContext oldcontext;
    bool        allow_jit;

    Assert(nbuckets > 0);

    /* Limit initial table size request to not more than work_mem */
    nbuckets = Min(nbuckets, (long) ((work_mem * 1024L) / entrysize));

    oldcontext = MemoryContextSwitchTo(metacxt);

    hashtable = (TupleHashTable) palloc(sizeof(TupleHashTableData));

    hashtable->numCols = numCols;
    hashtable->keyColIdx = keyColIdx;
    hashtable->tab_hash_funcs = hashfunctions;
    hashtable->tab_collations = collations;
    hashtable->tablecxt = tablecxt;
    hashtable->tempcxt = tempcxt;
    hashtable->entrysize = entrysize;
    hashtable->tableslot = NULL;    /* will be made on first lookup */
    hashtable->inputslot = NULL;
    hashtable->in_hash_funcs = NULL;
    hashtable->cur_eq_func = NULL;

    /*
     * If parallelism is in use, even if the master backend is performing the
     * scan itself, we don't want to create the hashtable exactly the same way
     * in all workers. As hashtables are iterated over in keyspace-order,
     * doing so in all processes in the same way is likely to lead to
     * "unbalanced" hashtables when the table size initially is
     * underestimated.
     */
    if (use_variable_hash_iv)
        hashtable->hash_iv = murmurhash32(ParallelWorkerNumber);
    else
        hashtable->hash_iv = 0;

    hashtable->hashtab = tuplehash_create(metacxt, nbuckets, hashtable);

    /*
     * We copy the input tuple descriptor just for safety --- we assume all
     * input tuples will have equivalent descriptors.
     */
    hashtable->tableslot = MakeSingleTupleTableSlot(CreateTupleDescCopy(inputDesc),
                                                    &TTSOpsMinimalTuple);

    /*
     * If the old reset interface is used (i.e. BuildTupleHashTable, rather
     * than BuildTupleHashTableExt), allowing JIT would lead to the generated
     * functions to a) live longer than the query b) be re-generated each time
     * the table is being reset. Therefore prevent JIT from being used in that
     * case, by not providing a parent node (which prevents accessing the
     * JitContext in the EState).
     */
    allow_jit = metacxt != tablecxt;

    /* build comparator for all columns */
    /* XXX: should we support non-minimal tuples for the inputslot? */
    hashtable->tab_eq_func = ExecBuildGroupingEqual(inputDesc, inputDesc,
                                                    &TTSOpsMinimalTuple, &TTSOpsMinimalTuple,
                                                    numCols,
                                                    keyColIdx, eqfuncoids, collations,
                                                    allow_jit ? parent : NULL);

    /*
     * While not pretty, it's ok to not shut down this context, but instead
     * rely on the containing memory context being reset, as
     * ExecBuildGroupingEqual() only builds a very simple expression calling
     * functions (i.e. nothing that'd employ RegisterExprContextCallback()).
     */
    hashtable->exprcontext = CreateStandaloneExprContext();

    MemoryContextSwitchTo(oldcontext);

    return hashtable;
}

/*
 * BuildTupleHashTable is a backwards-compatibilty wrapper for
 * BuildTupleHashTableExt(), that allocates the hashtable's metadata in
 * tablecxt. Note that hashtables created this way cannot be reset leak-free
 * with ResetTupleHashTable().
 */
TupleHashTable
BuildTupleHashTable(PlanState *parent,
                    TupleDesc inputDesc,
                    int numCols, AttrNumber *keyColIdx,
                    const Oid *eqfuncoids,
                    FmgrInfo *hashfunctions,
                    Oid *collations,
                    long nbuckets, Size additionalsize,
                    MemoryContext tablecxt,
                    MemoryContext tempcxt,
                    bool use_variable_hash_iv)
{
    return BuildTupleHashTableExt(parent,
                                  inputDesc,
                                  numCols, keyColIdx,
                                  eqfuncoids,
                                  hashfunctions,
                                  collations,
                                  nbuckets, additionalsize,
                                  tablecxt,
                                  tablecxt,
                                  tempcxt,
                                  use_variable_hash_iv);
}

/*
 * Reset contents of the hashtable to be empty, preserving all the non-content
 * state. Note that the tablecxt passed to BuildTupleHashTableExt() should
 * also be reset, otherwise there will be leaks.
 */
void
ResetTupleHashTable(TupleHashTable hashtable)
{
    tuplehash_reset(hashtable->hashtab);
}

/*
 * Find or create a hashtable entry for the tuple group containing the
 * given tuple.  The tuple must be the same type as the hashtable entries.
 *
 * If isnew is NULL, we do not create new entries; we return NULL if no
 * match is found.
 *
 * If isnew isn't NULL, then a new entry is created if no existing entry
 * matches.  On return, *isnew is true if the entry is newly created,
 * false if it existed already.  ->additional_data in the new entry has
 * been zeroed.
 */
TupleHashEntry
LookupTupleHashEntry(TupleHashTable hashtable, TupleTableSlot *slot,
                     bool *isnew)
{
    TupleHashEntryData *entry;
    MemoryContext oldContext;
    bool        found;
    MinimalTuple key;

    /* Need to run the hash functions in short-lived context */
    oldContext = MemoryContextSwitchTo(hashtable->tempcxt);

    /* set up data needed by hash and match functions */
    hashtable->inputslot = slot;
    hashtable->in_hash_funcs = hashtable->tab_hash_funcs;
    hashtable->cur_eq_func = hashtable->tab_eq_func;

    key = NULL;                 /* flag to reference inputslot */

    if (isnew)
    {
        entry = tuplehash_insert(hashtable->hashtab, key, &found);

        if (found)
        {
            /* found pre-existing entry */
            *isnew = false;
        }
        else
        {
            /* created new entry */
            *isnew = true;
            /* zero caller data */
            entry->additional = NULL;
            MemoryContextSwitchTo(hashtable->tablecxt);
            /* Copy the first tuple into the table context */
            entry->firstTuple = ExecCopySlotMinimalTuple(slot);
        }
    }
    else
    {
        entry = tuplehash_lookup(hashtable->hashtab, key);
    }

    MemoryContextSwitchTo(oldContext);

    return entry;
}

/*
 * Search for a hashtable entry matching the given tuple.  No entry is
 * created if there's not a match.  This is similar to the non-creating
 * case of LookupTupleHashEntry, except that it supports cross-type
 * comparisons, in which the given tuple is not of the same type as the
 * table entries.  The caller must provide the hash functions to use for
 * the input tuple, as well as the equality functions, since these may be
 * different from the table's internal functions.
 */
TupleHashEntry
FindTupleHashEntry(TupleHashTable hashtable, TupleTableSlot *slot,
                   ExprState *eqcomp,
                   FmgrInfo *hashfunctions)
{
    TupleHashEntry entry;
    MemoryContext oldContext;
    MinimalTuple key;

    /* Need to run the hash functions in short-lived context */
    oldContext = MemoryContextSwitchTo(hashtable->tempcxt);

    /* Set up data needed by hash and match functions */
    hashtable->inputslot = slot;
    hashtable->in_hash_funcs = hashfunctions;
    hashtable->cur_eq_func = eqcomp;

    /* Search the hash table */
    key = NULL;                 /* flag to reference inputslot */
    entry = tuplehash_lookup(hashtable->hashtab, key);
    MemoryContextSwitchTo(oldContext);

    return entry;
}

/*
 * Compute the hash value for a tuple
 *
 * If tuple is NULL, use the input slot instead. This convention avoids the
 * need to materialize virtual input tuples unless they actually need to get
 * copied into the table.
 *
 * Also, the caller must select an appropriate memory context for running
 * the hash functions. (dynahash.c doesn't change CurrentMemoryContext.)
 */
static uint32
TupleHashTableHash(struct tuplehash_hash *tb, const MinimalTuple tuple)
{
    TupleHashTable hashtable = (TupleHashTable) tb->private_data;
    int         numCols = hashtable->numCols;
    AttrNumber *keyColIdx = hashtable->keyColIdx;
    uint32      hashkey = hashtable->hash_iv;
    TupleTableSlot *slot;
    FmgrInfo   *hashfunctions;
    int         i;

    if (tuple == NULL)
    {
        /* Process the current input tuple for the table */
        slot = hashtable->inputslot;
        hashfunctions = hashtable->in_hash_funcs;
    }
    else
    {
        /*
         * Process a tuple already stored in the table.
         *
         * (this case never actually occurs due to the way simplehash.h is
         * used, as the hash-value is stored in the entries)
         */
        slot = hashtable->tableslot;
        ExecStoreMinimalTuple(tuple, slot, false);
        hashfunctions = hashtable->tab_hash_funcs;
    }

    for (i = 0; i < numCols; i++)
    {
        AttrNumber  att = keyColIdx[i];
        Datum       attr;
        bool        isNull;

        /* rotate hashkey left 1 bit at each step */
        hashkey = (hashkey << 1) | ((hashkey & 0x80000000) ? 1 : 0);

        attr = slot_getattr(slot, att, &isNull);

        if (!isNull)            /* treat nulls as having hash key 0 */
        {
            uint32      hkey;

            hkey = DatumGetUInt32(FunctionCall1Coll(&hashfunctions[i],
                                                    hashtable->tab_collations[i],
                                                    attr));
            hashkey ^= hkey;
        }
    }

    /*
     * The way hashes are combined above, among each other and with the IV,
     * doesn't lead to good bit perturbation. As the IV's goal is to lead to
     * achieve that, perform a round of hashing of the combined hash -
     * resulting in near perfect perturbation.
     */
    return murmurhash32(hashkey);
}

/*
 * See whether two tuples (presumably of the same hash value) match
 */
static int
TupleHashTableMatch(struct tuplehash_hash *tb, const MinimalTuple tuple1, const MinimalTuple tuple2)
{
    TupleTableSlot *slot1;
    TupleTableSlot *slot2;
    TupleHashTable hashtable = (TupleHashTable) tb->private_data;
    ExprContext *econtext = hashtable->exprcontext;

    /*
     * We assume that simplehash.h will only ever call us with the first
     * argument being an actual table entry, and the second argument being
     * LookupTupleHashEntry's dummy TupleHashEntryData.  The other direction
     * could be supported too, but is not currently required.
     */
    Assert(tuple1 != NULL);
    slot1 = hashtable->tableslot;
    ExecStoreMinimalTuple(tuple1, slot1, false);
    Assert(tuple2 == NULL);
    slot2 = hashtable->inputslot;

    /* For crosstype comparisons, the inputslot must be first */
    econtext->ecxt_innertuple = slot2;
    econtext->ecxt_outertuple = slot1;
    return !ExecQualAndReset(hashtable->cur_eq_func, econtext);
}