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Commite246b3d

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Add a fast pre-check for equality of equal-length strings.
Testing reveals that that doing a memcmp() before the strcoll() costspractically nothing, at least on the systems we tested, and it speedsup sorts containing many equal strings significatly.Peter Geoghegan. Review by myself and Heikki Linnakangas. Commentsrewritten by me.
1 parent491c029 commite246b3d

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‎src/backend/utils/adt/varlena.c

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Original file line numberDiff line numberDiff line change
@@ -1405,6 +1405,18 @@ varstr_cmp(char *arg1, int len1, char *arg2, int len2, Oid collid)
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#endif
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}
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/*
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* memcmp() can't tell us which of two unequal strings sorts first, but
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* it's a cheap way to tell if they're equal. Testing shows that
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* memcmp() followed by strcoll() is only trivially slower than
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* strcoll() by itself, so we don't lose much if this doesn't work out
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* very often, and if it does - for example, because there are many
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* equal strings in the input - then we win big by avoiding expensive
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* collation-aware comparisons.
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*/
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if (len1==len2&&memcmp(arg1,arg2,len1)==0)
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return0;
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#ifdefWIN32
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/* Win32 does not have UTF-8, so we need to map to UTF-16 */
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if (GetDatabaseEncoding()==PG_UTF8)
@@ -1842,6 +1854,13 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
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len1=VARSIZE_ANY_EXHDR(arg1);
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len2=VARSIZE_ANY_EXHDR(arg2);
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/* Fast pre-check for equality, as discussed in varstr_cmp() */
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if (len1==len2&&memcmp(a1p,a2p,len1)==0)
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{
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result=0;
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gotodone;
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}
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if (len1 >=tss->buflen1)
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{
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pfree(tss->buf1);
@@ -1875,6 +1894,7 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
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if (result==0)
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result=strcmp(tss->buf1,tss->buf2);
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done:
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/* We can't afford to leak memory here. */
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if (PointerGetDatum(arg1)!=x)
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pfree(arg1);

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