diff options
Diffstat (limited to 'src/include/utils/sortsupport.h')
| -rw-r--r-- | src/include/utils/sortsupport.h | 37 |
1 files changed, 19 insertions, 18 deletions
diff --git a/src/include/utils/sortsupport.h b/src/include/utils/sortsupport.h index 42586305563..e890503d31f 100644 --- a/src/include/utils/sortsupport.h +++ b/src/include/utils/sortsupport.h @@ -116,24 +116,25 @@ typedef struct SortSupportData * * This allows opclass authors to supply a conversion routine, used to * create an alternative representation of the underlying type (an - * "abbreviated key"). Typically, this representation is an ad-hoc, - * pass-by-value Datum format that only the opclass has knowledge of. An - * alternative comparator, used only with this alternative representation - * must also be provided (which is assigned to "comparator"). This - * representation is a simple approximation of the original Datum. It - * must be possible to compare datums of this representation with each - * other using the supplied alternative comparator, and have any non-zero - * return value be a reliable proxy for what a proper comparison would - * indicate. Returning zero from the alternative comparator does not - * indicate equality, as with a conventional support routine 1, though -- - * it indicates that it wasn't possible to determine how the two - * abbreviated values compared. A proper comparison, using - * "abbrev_full_comparator"/ ApplySortAbbrevFullComparator() is therefore - * required. In many cases this results in most or all comparisons only - * using the cheap alternative comparison func, which is typically - * implemented as code that compiles to just a few CPU instructions. CPU - * cache miss penalties are expensive; to get good overall performance, - * sort infrastructure must heavily weigh cache performance. + * "abbreviated key"). This representation must be pass-by-value and + * typically will use some ad-hoc format that only the opclass has + * knowledge of. An alternative comparator, used only with this + * alternative representation must also be provided (which is assigned to + * "comparator"). This representation is a simple approximation of the + * original Datum. It must be possible to compare datums of this + * representation with each other using the supplied alternative + * comparator, and have any non-zero return value be a reliable proxy for + * what a proper comparison would indicate. Returning zero from the + * alternative comparator does not indicate equality, as with a + * conventional support routine 1, though -- it indicates that it wasn't + * possible to determine how the two abbreviated values compared. A + * proper comparison, using "abbrev_full_comparator"/ + * ApplySortAbbrevFullComparator() is therefore required. In many cases + * this results in most or all comparisons only using the cheap + * alternative comparison func, which is typically implemented as code + * that compiles to just a few CPU instructions. CPU cache miss penalties + * are expensive; to get good overall performance, sort infrastructure + * must heavily weigh cache performance. * * Opclass authors must consider the final cardinality of abbreviated keys * when devising an encoding scheme. It's possible for a strategy to work |