Add sort support routine for the UUID data type.

This introduces a simple encoding scheme to produce abbreviated keys:
pack as many bytes of each UUID as will fit into a Datum.  On
little-endian machines, a byteswap is also performed; the abbreviated
comparator can therefore just consist of a simple 3-way unsigned integer
comparison.

The purpose of this change is to speed up sorting data on a column
of type UUID.

Peter Geoghegan

src/backend/utils/adt/uuid.c

@@ -14,8 +14,12 @@
 #include "postgres.h"
 
 #include "access/hash.h"
+#include "lib/hyperloglog.h"
 #include "libpq/pqformat.h"
+#include "port/pg_bswap.h"
 #include "utils/builtins.h"
+#include "utils/guc.h"
+#include "utils/sortsupport.h"
 #include "utils/uuid.h"
 
 /* uuid size in bytes */
@@ -27,8 +31,21 @@ struct pg_uuid_t
 	unsigned char data[UUID_LEN];
 };
 
+/* sortsupport for uuid */
+typedef struct
+{
+	int64		input_count;	/* number of non-null values seen */
+	bool		estimating;		/* true if estimating cardinality */
+
+	hyperLogLogState abbr_card; /* cardinality estimator */
+} uuid_sortsupport_state;
+
 static void string_to_uuid(const char *source, pg_uuid_t *uuid);
 static int	uuid_internal_cmp(const pg_uuid_t *arg1, const pg_uuid_t *arg2);
+static int	uuid_fast_cmp(Datum x, Datum y, SortSupport ssup);
+static int	uuid_cmp_abbrev(Datum x, Datum y, SortSupport ssup);
+static bool	uuid_abbrev_abort(int memtupcount, SortSupport ssup);
+static Datum	uuid_abbrev_convert(Datum original, SortSupport ssup);
 
 Datum
 uuid_in(PG_FUNCTION_ARGS)
@@ -222,6 +239,176 @@ uuid_cmp(PG_FUNCTION_ARGS)
 	PG_RETURN_INT32(uuid_internal_cmp(arg1, arg2));
 }
 
+/*
+ * Sort support strategy routine
+ */
+Datum
+uuid_sortsupport(PG_FUNCTION_ARGS)
+{
+	SortSupport	ssup = (SortSupport) PG_GETARG_POINTER(0);
+
+	ssup->comparator = uuid_fast_cmp;
+	ssup->ssup_extra = NULL;
+
+	if (ssup->abbreviate)
+	{
+		uuid_sortsupport_state	   *uss;
+		MemoryContext				oldcontext;
+
+		oldcontext = MemoryContextSwitchTo(ssup->ssup_cxt);
+
+		uss = palloc(sizeof(uuid_sortsupport_state));
+		uss->input_count = 0;
+		uss->estimating = true;
+		initHyperLogLog(&uss->abbr_card, 10);
+
+		ssup->ssup_extra = uss;
+
+		ssup->comparator = uuid_cmp_abbrev;
+		ssup->abbrev_converter = uuid_abbrev_convert;
+		ssup->abbrev_abort = uuid_abbrev_abort;
+		ssup->abbrev_full_comparator = uuid_fast_cmp;
+
+		MemoryContextSwitchTo(oldcontext);
+	}
+
+	PG_RETURN_VOID();
+}
+
+/*
+ * SortSupport comparison func
+ */
+static int
+uuid_fast_cmp(Datum x, Datum y, SortSupport ssup)
+{
+	pg_uuid_t  *arg1 = DatumGetUUIDP(x);
+	pg_uuid_t  *arg2 = DatumGetUUIDP(y);
+
+	return uuid_internal_cmp(arg1, arg2);
+}
+
+/*
+ * Abbreviated key comparison func
+ */
+static int
+uuid_cmp_abbrev(Datum x, Datum y, SortSupport ssup)
+{
+	if (x > y)
+		return 1;
+	else if (x == y)
+		return 0;
+	else
+		return -1;
+}
+
+/*
+ * Callback for estimating effectiveness of abbreviated key optimization.
+ *
+ * We pay no attention to the cardinality of the non-abbreviated data, because
+ * there is no equality fast-path within authoritative uuid comparator.
+ */
+static bool
+uuid_abbrev_abort(int memtupcount, SortSupport ssup)
+{
+	uuid_sortsupport_state	   *uss = ssup->ssup_extra;
+	double						abbr_card;
+
+	if (memtupcount < 10000 || uss->input_count < 10000 || !uss->estimating)
+		return false;
+
+	abbr_card = estimateHyperLogLog(&uss->abbr_card);
+
+	/*
+	 * If we have >100k distinct values, then even if we were sorting many
+	 * billion rows we'd likely still break even, and the penalty of undoing
+	 * that many rows of abbrevs would probably not be worth it.  Stop even
+	 * counting at that point.
+	 */
+	if (abbr_card > 100000.0)
+	{
+#ifdef TRACE_SORT
+		if (trace_sort)
+			elog(LOG,
+				 "uuid_abbrev: estimation ends at cardinality %f"
+				 " after " INT64_FORMAT " values (%d rows)",
+				 abbr_card, uss->input_count, memtupcount);
+#endif
+		uss->estimating = false;
+		return false;
+	}
+
+	/*
+	 * Target minimum cardinality is 1 per ~2k of non-null inputs.  0.5 row
+	 * fudge factor allows us to abort earlier on genuinely pathological data
+	 * where we've had exactly one abbreviated value in the first 2k (non-null)
+	 * rows.
+	 */
+	if (abbr_card < uss->input_count / 2000.0 + 0.5)
+	{
+#ifdef TRACE_SORT
+		if (trace_sort)
+			elog(LOG,
+				 "uuid_abbrev: aborting abbreviation at cardinality %f"
+				 " below threshold %f after " INT64_FORMAT " values (%d rows)",
+				 abbr_card, uss->input_count / 2000.0 + 0.5, uss->input_count,
+				 memtupcount);
+#endif
+		return true;
+	}
+
+#ifdef TRACE_SORT
+	if (trace_sort)
+		elog(LOG,
+			 "uuid_abbrev: cardinality %f after " INT64_FORMAT
+			 " values (%d rows)", abbr_card, uss->input_count, memtupcount);
+#endif
+
+	return false;
+}
+
+/*
+ * Conversion routine for sortsupport.  Converts original uuid representation
+ * to abbreviated key representation.  Our encoding strategy is simple -- pack
+ * the first `sizeof(Datum)` bytes of uuid data into a Datum (on little-endian
+ * machines, the bytes are stored in reverse order), and treat it as an
+ * unsigned integer.
+ */
+static Datum
+uuid_abbrev_convert(Datum original, SortSupport ssup)
+{
+	uuid_sortsupport_state	   *uss = ssup->ssup_extra;
+	pg_uuid_t				   *authoritative = DatumGetUUIDP(original);
+	Datum						res;
+
+	memcpy((char *) &res, authoritative->data, sizeof(Datum));
+	uss->input_count += 1;
+
+	if (uss->estimating)
+	{
+		uint32		tmp;
+
+#if SIZEOF_DATUM == 8
+		tmp = (uint32) res ^ (uint32) ((uint64) res >> 32);
+#else							/* SIZEOF_DATUM != 8 */
+		tmp = (uint32) res;
+#endif
+
+		addHyperLogLog(&uss->abbr_card, DatumGetUInt32(hash_uint32(tmp)));
+	}
+
+	/*
+	 * Byteswap on little-endian machines.
+	 *
+	 * This is needed so that uuid_cmp_abbrev() (an unsigned integer 3-way
+	 * comparator) works correctly on all platforms.  If we didn't do this, the
+	 * comparator would have to call memcmp() with a pair of pointers to the
+	 * first byte of each abbreviated key, which is slower.
+	 */
+	res = DatumBigEndianToNative(res);
+
+	return res;
+}
+
 /* hash index support */
 Datum
 uuid_hash(PG_FUNCTION_ARGS)

src/include/catalog/pg_amproc.h

@@ -134,6 +134,7 @@ DATA(insert (	2233   703 703 1  380 ));
 DATA(insert (	2234   704 704 1  381 ));
 DATA(insert (	2789   27 27 1 2794 ));
 DATA(insert (	2968   2950 2950 1 2960 ));
+DATA(insert (	2968   2950 2950 2 3300 ));
 DATA(insert (	2994   2249 2249 1 2987 ));
 DATA(insert (	3194   2249 2249 1 3187 ));
 DATA(insert (	3253   3220 3220 1 3251 ));

src/include/catalog/pg_proc.h

@@ -4467,6 +4467,8 @@ DATA(insert OID = 2958 (  uuid_gt		   PGNSP PGUID 12 1 0 0 0 f f f t t f i s 2 0
 DATA(insert OID = 2959 (  uuid_ne		   PGNSP PGUID 12 1 0 0 0 f f f t t f i s 2 0 16 "2950 2950" _null_ _null_ _null_ _null_ _null_ uuid_ne _null_ _null_ _null_ ));
 DATA(insert OID = 2960 (  uuid_cmp		   PGNSP PGUID 12 1 0 0 0 f f f f t f i s 2 0 23 "2950 2950" _null_ _null_ _null_ _null_ _null_ uuid_cmp _null_ _null_ _null_ ));
 DESCR("less-equal-greater");
+DATA(insert OID = 3300 (  uuid_sortsupport PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 2278 "2281" _null_ _null_ _null_ _null_ _null_ uuid_sortsupport _null_ _null_ _null_ ));
+DESCR("sort support");
 DATA(insert OID = 2961 (  uuid_recv		   PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 2950 "2281" _null_ _null_ _null_ _null_ _null_ uuid_recv _null_ _null_ _null_ ));
 DESCR("I/O");
 DATA(insert OID = 2962 (  uuid_send		   PGNSP PGUID 12 1 0 0 0 f f f f t f i s 1 0 17 "2950" _null_ _null_ _null_ _null_ _null_ uuid_send _null_ _null_ _null_ ));

src/include/utils/builtins.h

@@ -1174,6 +1174,7 @@ extern Datum uuid_ge(PG_FUNCTION_ARGS);
 extern Datum uuid_gt(PG_FUNCTION_ARGS);
 extern Datum uuid_ne(PG_FUNCTION_ARGS);
 extern Datum uuid_cmp(PG_FUNCTION_ARGS);
+extern Datum uuid_sortsupport(PG_FUNCTION_ARGS);
 extern Datum uuid_hash(PG_FUNCTION_ARGS);
 
 /* windowfuncs.c */