From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 9 Oct 2015 23:03:44 +0000 (-0400)
Subject: Speed up text sorts where the same strings occur multiple times.
X-Git-Tag: REL9_6_BETA1~1219
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=0e57b4d8bd9674adaf5747421b3255b85e385534;p=postgresql

Speed up text sorts where the same strings occur multiple times.

Cache strxfrm() blobs across calls made to the text SortSupport
abbreviation routine.  This can speed up sorting if the same string
needs to be abbreviated many times in a row.

Also, cache the result of the previous strcoll() comparison, so that
if we're asked to compare the same strings agin, we do need to call
strcoll() again.

Perhaps surprisingly, these optimizations don't seem to hurt even when
they don't help.  memcmp() is really cheap compared to strcoll() or
strxfrm().

Peter Geoghegan, reviewed by me.
---

diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 49a4898987..d545c34b6b 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -62,6 +62,9 @@ typedef struct
 	char	   *buf2;			/* 2nd string, or abbreviation strxfrm() buf */
 	int			buflen1;
 	int			buflen2;
+	int			last_len1;		/* Length of last buf1 string/strxfrm() blob */
+	int			last_len2;		/* Length of last buf2 string/strxfrm() blob */
+	int			last_returned;	/* Last comparison result (cache) */
 	bool		collate_c;
 	hyperLogLogState abbr_card; /* Abbreviated key cardinality state */
 	hyperLogLogState full_card; /* Full key cardinality state */
@@ -1832,9 +1835,20 @@ btsortsupport_worker(SortSupport ssup, Oid collid)
 		tss->buflen1 = TEXTBUFLEN;
 		tss->buf2 = palloc(TEXTBUFLEN);
 		tss->buflen2 = TEXTBUFLEN;
+		/* Start with invalid values */
+		tss->last_len1 = -1;
+		tss->last_len2 = -1;
 #ifdef HAVE_LOCALE_T
 		tss->locale = locale;
 #endif
+		/*
+		 * To avoid somehow confusing a strxfrm() blob and an original string
+		 * within bttextfastcmp_locale(), initialize last returned cache to a
+		 * sentinel value.  A platform-specific actual strcoll() return value
+		 * of INT_MIN seems unlikely, but if that occurs it will not cause
+		 * wrong answers.
+		 */
+		tss->last_returned = INT_MIN;
 		tss->collate_c = collate_c;
 		ssup->ssup_extra = tss;
 
@@ -1897,6 +1911,7 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 {
 	text	   *arg1 = DatumGetTextPP(x);
 	text	   *arg2 = DatumGetTextPP(y);
+	bool		arg1_match;
 	TextSortSupport *tss = (TextSortSupport *) ssup->ssup_extra;
 
 	/* working state */
@@ -1915,6 +1930,11 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 	/* Fast pre-check for equality, as discussed in varstr_cmp() */
 	if (len1 == len2 && memcmp(a1p, a2p, len1) == 0)
 	{
+		/*
+		 * No change in buf1 or buf2 contents, so avoid changing last_len1 or
+		 * last_len2.  Existing contents of buffers might still be used by next
+		 * call.
+		 */
 		result = 0;
 		goto done;
 	}
@@ -1932,10 +1952,43 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 		tss->buf2 = MemoryContextAlloc(ssup->ssup_cxt, tss->buflen2);
 	}
 
-	memcpy(tss->buf1, a1p, len1);
-	tss->buf1[len1] = '\0';
-	memcpy(tss->buf2, a2p, len2);
-	tss->buf2[len2] = '\0';
+	/*
+	 * We're likely to be asked to compare the same strings repeatedly, and
+	 * memcmp() is so much cheaper than strcoll() that it pays to try to cache
+	 * comparisons, even though in general there is no reason to think that
+	 * that will work out (every text datum may be unique).  Caching does not
+	 * slow things down measurably when it doesn't work out, and can speed
+	 * things up by rather a lot when it does.  In part, this is because the
+	 * memcmp() compares data from cachelines that are needed in L1 cache even
+	 * when the last comparison's result cannot be reused.
+	 */
+	arg1_match = true;
+	if (len1 != tss->last_len1 || memcmp(tss->buf1, a1p, len1) != 0)
+	{
+		arg1_match = false;
+		memcpy(tss->buf1, a1p, len1);
+		tss->buf1[len1] = '\0';
+		tss->last_len1 = len1;
+	}
+
+	/*
+	 * If we're comparing the same two strings as last time, we can return the
+	 * same answer without calling strcoll() again.  This is more likely than
+	 * it seems (at least with moderate to low cardinality sets), because
+	 * quicksort compares the same pivot against many values.
+	 */
+	if (len2 != tss->last_len2 || memcmp(tss->buf2, a2p, len2) != 0)
+	{
+		memcpy(tss->buf2, a2p, len2);
+		tss->buf2[len2] = '\0';
+		tss->last_len2 = len2;
+	}
+	else if (arg1_match && tss->last_returned != INT_MIN)
+	{
+		/* Use result cached following last actual strcoll() call */
+		result = tss->last_returned;
+		goto done;
+	}
 
 #ifdef HAVE_LOCALE_T
 	if (tss->locale)
@@ -1952,6 +2005,8 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
 	if (result == 0)
 		result = strcmp(tss->buf1, tss->buf2);
 
+	/* Cache result, perhaps saving an expensive strcoll() call next time */
+	tss->last_returned = result;
 done:
 	/* We can't afford to leak memory here. */
 	if (PointerGetDatum(arg1) != x)
@@ -2034,9 +2089,19 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 			tss->buf1 = palloc(tss->buflen1);
 		}
 
+		/* Might be able to reuse strxfrm() blob from last call */
+		if (tss->last_len1 == len &&
+			memcmp(tss->buf1, authoritative_data, len) == 0)
+		{
+			memcpy(pres, tss->buf2, Min(sizeof(Datum), tss->last_len2));
+			/* No change affecting cardinality, so no hashing required */
+			goto done;
+		}
+
 		/* Just like strcoll(), strxfrm() expects a NUL-terminated string */
 		memcpy(tss->buf1, authoritative_data, len);
 		tss->buf1[len] = '\0';
+		tss->last_len1 = len;
 
 		for (;;)
 		{
@@ -2048,6 +2113,7 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 #endif
 				bsize = strxfrm(tss->buf2, tss->buf1, tss->buflen2);
 
+			tss->last_len2 = bsize;
 			if (bsize < tss->buflen2)
 				break;
 
@@ -2105,6 +2171,7 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
 
 	addHyperLogLog(&tss->abbr_card, hash);
 
+done:
 	/*
 	 * Byteswap on little-endian machines.
 	 *