diff options
Diffstat (limited to 'src/backend/utils/adt/selfuncs.c')
| -rw-r--r-- | src/backend/utils/adt/selfuncs.c | 37 |
1 files changed, 28 insertions, 9 deletions
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c index 107d85fa8b8..f5660b2c3cd 100644 --- a/src/backend/utils/adt/selfuncs.c +++ b/src/backend/utils/adt/selfuncs.c @@ -5701,13 +5701,23 @@ byte_increment(unsigned char *ptr, int len) * and "9" is seen as largest by the collation, and append that to the given * prefix before trying to find a string that compares as larger. * - * If we max out the righthand byte, truncate off the last character - * and start incrementing the next. For example, if "z" were the last - * character in the sort order, then we could produce "foo" as a - * string greater than "fonz". + * To search for a greater string, we repeatedly "increment" the rightmost + * character, using an encoding-specific character incrementer function. + * When it's no longer possible to increment the last character, we truncate + * off that character and start incrementing the next-to-rightmost. + * For example, if "z" were the last character in the sort order, then we + * could produce "foo" as a string greater than "fonz". * * This could be rather slow in the worst case, but in most cases we * won't have to try more than one or two strings before succeeding. + * + * Note that it's important for the character incrementer not to be too anal + * about producing every possible character code, since in some cases the only + * way to get a larger string is to increment a previous character position. + * So we don't want to spend too much time trying every possible character + * code at the last position. A good rule of thumb is to be sure that we + * don't try more than 256*K values for a K-byte character (and definitely + * not 256^K, which is what an exhaustive search would approach). */ Const * make_greater_string(const Const *str_const, FmgrInfo *ltproc, Oid collation) @@ -5779,17 +5789,19 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc, Oid collation) } } + /* Select appropriate character-incrementer function */ if (datatype == BYTEAOID) - charinc = &byte_increment; + charinc = byte_increment; else charinc = pg_database_encoding_character_incrementer(); + /* And search ... */ while (len > 0) { - int charlen; + int charlen; unsigned char *lastchar; - Const *workstr_const; + /* Identify the last character --- for bytea, just the last byte */ if (datatype == BYTEAOID) charlen = 1; else @@ -5799,9 +5811,15 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc, Oid collation) /* * Try to generate a larger string by incrementing the last character * (for BYTEA, we treat each byte as a character). + * + * Note: the incrementer function is expected to return true if it's + * generated a valid-per-the-encoding new character, otherwise false. + * The contents of the character on false return are unspecified. */ - if (charinc(lastchar, charlen)) + while (charinc(lastchar, charlen)) { + Const *workstr_const; + if (datatype == BYTEAOID) workstr_const = string_to_bytea_const(workstr, len); else @@ -5825,7 +5843,8 @@ make_greater_string(const Const *str_const, FmgrInfo *ltproc, Oid collation) } /* - * Truncate off the last character or byte. + * No luck here, so truncate off the last character and try to + * increment the next one. */ len -= charlen; workstr[len] = '\0'; |