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Commitbfb54ff

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Make abbreviated key comparisons for text a bit cheaper.
If we do some byte-swapping while abbreviating, we can do comparisonsusing integer arithmetic rather than memcmp.Peter Geoghegan, reviewed and slightly revised by me.
1 parentdb0f6ca commitbfb54ff

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2 files changed

+44
-11
lines changed

2 files changed

+44
-11
lines changed

‎src/backend/utils/adt/varlena.c

Lines changed: 20 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -26,6 +26,7 @@
2626
#include"libpq/pqformat.h"
2727
#include"miscadmin.h"
2828
#include"parser/scansup.h"
29+
#include"port/pg_bswap.h"
2930
#include"regex/regex.h"
3031
#include"utils/builtins.h"
3132
#include"utils/bytea.h"
@@ -1967,25 +1968,25 @@ bttextfastcmp_locale(Datum x, Datum y, SortSupport ssup)
19671968
staticint
19681969
bttextcmp_abbrev(Datumx,Datumy,SortSupportssup)
19691970
{
1970-
char*a= (char*)&x;
1971-
char*b= (char*)&y;
1972-
intresult;
1973-
1974-
result=memcmp(a,b,sizeof(Datum));
1975-
19761971
/*
1977-
* Whenresult = 0, the core system will call bttextfastcmp_c() or
1972+
* When0 is returned, the core system will call bttextfastcmp_c() or
19781973
* bttextfastcmp_locale(). Even a strcmp() on two non-truncated strxfrm()
19791974
* blobs cannot indicate *equality* authoritatively, for the same reason
19801975
* that there is a strcoll() tie-breaker call to strcmp() in varstr_cmp().
19811976
*/
1982-
returnresult;
1977+
if (x>y)
1978+
return1;
1979+
elseif (x==y)
1980+
return0;
1981+
else
1982+
return-1;
19831983
}
19841984

19851985
/*
19861986
* Conversion routine for sortsupport. Converts original text to abbreviated
19871987
* key representation. Our encoding strategy is simple -- pack the first 8
1988-
* bytes of a strxfrm() blob into a Datum.
1988+
* bytes of a strxfrm() blob into a Datum (on little-endian machines, the 8
1989+
* bytes are stored in reverse order), and treat it as an unsigned integer.
19891990
*/
19901991
staticDatum
19911992
bttext_abbrev_convert(Datumoriginal,SortSupportssup)
@@ -2104,6 +2105,16 @@ bttext_abbrev_convert(Datum original, SortSupport ssup)
21042105

21052106
addHyperLogLog(&tss->abbr_card,hash);
21062107

2108+
/*
2109+
* Byteswap on little-endian machines.
2110+
*
2111+
* This is needed so that bttextcmp_abbrev() (an unsigned integer 3-way
2112+
* comparator) works correctly on all platforms. If we didn't do this,
2113+
* the comparator would have to call memcmp() with a pair of pointers to
2114+
* the first byte of each abbreviated key, which is slower.
2115+
*/
2116+
res=DatumBigEndianToNative(res);
2117+
21072118
/* Don't leak memory here */
21082119
if (PointerGetDatum(authoritative)!=original)
21092120
pfree(authoritative);

‎src/include/port/pg_bswap.h

Lines changed: 24 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -28,7 +28,7 @@
2828
((x << 8) & 0x00ff0000) | \
2929
((x >> 8) & 0x0000ff00) | \
3030
((x >> 24) & 0x000000ff))
31-
#endif/* HAVE__BUILTIN_BSWAP32 */
31+
#endif/* HAVE__BUILTIN_BSWAP32 */
3232

3333
#ifdefHAVE__BUILTIN_BSWAP64
3434
#defineBSWAP64(x) __builtin_bswap64(x)
@@ -41,6 +41,28 @@
4141
((x >> 24) & 0x0000000000ff0000UL) | \
4242
((x >> 40) & 0x000000000000ff00UL) | \
4343
((x >> 56) & 0x00000000000000ffUL))
44-
#endif/* HAVE__BUILTIN_BSWAP64 */
44+
#endif/* HAVE__BUILTIN_BSWAP64 */
45+
46+
/*
47+
* Rearrange the bytes of a Datum from big-endian order into the native byte
48+
* order. On big-endian machines, this does nothing at all. Note that the C
49+
* type Datum is an unsigned integer type on all platforms.
50+
*
51+
* One possible application of the DatumBigEndianToNative() macro is to make
52+
* bitwise comparisons cheaper. A simple 3-way comparison of Datums
53+
* transformed by the macro (based on native, unsigned comparisons) will return
54+
* the same result as a memcmp() of the corresponding original Datums, but can
55+
* be much cheaper. It's generally safe to do this on big-endian systems
56+
* without any special transformation occurring first.
57+
*/
58+
#ifdefWORDS_BIGENDIAN
59+
#defineDatumBigEndianToNative(x)(x)
60+
#else/* !WORDS_BIGENDIAN */
61+
#ifSIZEOF_DATUM==8
62+
#defineDatumBigEndianToNative(x)BSWAP64(x)
63+
#else/* SIZEOF_DATUM != 8 */
64+
#defineDatumBigEndianToNative(x)BSWAP32(x)
65+
#endif/* SIZEOF_DATUM == 8 */
66+
#endif/* WORDS_BIGENDIAN */
4567

4668
#endif/* PG_BSWAP_H */

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