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Comment improvements for abbreviated keys.
Peter Geoghegan and Robert Haas
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@ -930,7 +930,14 @@ tuplesort_begin_datum(Oid datumType, Oid sortOperator, Oid sortCollation,
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state->sortKeys->ssup_collation = sortCollation;
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state->sortKeys->ssup_nulls_first = nullsFirstFlag;
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/* abbreviation is possible here only for by-reference types */
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/*
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* Abbreviation is possible here only for by-reference types. In theory,
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* a pass-by-value datatype could have an abbreviated form that is cheaper
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* to compare. In a tuple sort, we could support that, because we can
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* always extract the original datum from the tuple is needed. Here, we
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* can't, because a datum sort only stores a single copy of the datum;
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* the "tuple" field of each sortTuple is NULL.
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*/
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state->sortKeys->abbreviate = !typbyval;
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PrepareSortSupportFromOrderingOp(sortOperator, state->sortKeys);
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@ -1271,7 +1278,7 @@ tuplesort_putindextuplevalues(Tuplesortstate *state, Relation rel,
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* Store ordinary Datum representation, or NULL value. If there is a
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* converter it won't expect NULL values, and cost model is not
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* required to account for NULL, so in that case we avoid calling
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* converter and just set datum1 to "void" representation (to be
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* converter and just set datum1 to zeroed representation (to be
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* consistent).
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*/
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stup.datum1 = original;
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@ -3155,7 +3162,7 @@ copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup)
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* Store ordinary Datum representation, or NULL value. If there is a
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* converter it won't expect NULL values, and cost model is not
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* required to account for NULL, so in that case we avoid calling
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* converter and just set datum1 to "void" representation (to be
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* converter and just set datum1 to zeroed representation (to be
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* consistent).
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*/
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stup->datum1 = original;
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@ -3397,7 +3404,7 @@ copytup_cluster(Tuplesortstate *state, SortTuple *stup, void *tup)
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* Store ordinary Datum representation, or NULL value. If there is a
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* converter it won't expect NULL values, and cost model is not
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* required to account for NULL, so in that case we avoid calling
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* converter and just set datum1 to "void" representation (to be
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* converter and just set datum1 to zeroed representation (to be
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* consistent).
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*/
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stup->datum1 = original;
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@ -3701,7 +3708,7 @@ copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup)
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* Store ordinary Datum representation, or NULL value. If there is a
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* converter it won't expect NULL values, and cost model is not
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* required to account for NULL, so in that case we avoid calling
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* converter and just set datum1 to "void" representation (to be
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* converter and just set datum1 to zeroed representation (to be
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* consistent).
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*/
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stup->datum1 = original;
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@ -116,24 +116,25 @@ typedef struct SortSupportData
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*
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* This allows opclass authors to supply a conversion routine, used to
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* create an alternative representation of the underlying type (an
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* "abbreviated key"). Typically, this representation is an ad-hoc,
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* pass-by-value Datum format that only the opclass has knowledge of. An
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* alternative comparator, used only with this alternative representation
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* must also be provided (which is assigned to "comparator"). This
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* representation is a simple approximation of the original Datum. It
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* must be possible to compare datums of this representation with each
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* other using the supplied alternative comparator, and have any non-zero
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* return value be a reliable proxy for what a proper comparison would
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* indicate. Returning zero from the alternative comparator does not
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* indicate equality, as with a conventional support routine 1, though --
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* it indicates that it wasn't possible to determine how the two
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* abbreviated values compared. A proper comparison, using
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* "abbrev_full_comparator"/ ApplySortAbbrevFullComparator() is therefore
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* required. In many cases this results in most or all comparisons only
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* using the cheap alternative comparison func, which is typically
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* implemented as code that compiles to just a few CPU instructions. CPU
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* cache miss penalties are expensive; to get good overall performance,
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* sort infrastructure must heavily weigh cache performance.
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* "abbreviated key"). This representation must be pass-by-value and
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* typically will use some ad-hoc format that only the opclass has
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* knowledge of. An alternative comparator, used only with this
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* alternative representation must also be provided (which is assigned to
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* "comparator"). This representation is a simple approximation of the
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* original Datum. It must be possible to compare datums of this
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* representation with each other using the supplied alternative
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* comparator, and have any non-zero return value be a reliable proxy for
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* what a proper comparison would indicate. Returning zero from the
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* alternative comparator does not indicate equality, as with a
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* conventional support routine 1, though -- it indicates that it wasn't
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* possible to determine how the two abbreviated values compared. A
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* proper comparison, using "abbrev_full_comparator"/
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* ApplySortAbbrevFullComparator() is therefore required. In many cases
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* this results in most or all comparisons only using the cheap
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* alternative comparison func, which is typically implemented as code
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* that compiles to just a few CPU instructions. CPU cache miss penalties
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* are expensive; to get good overall performance, sort infrastructure
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* must heavily weigh cache performance.
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*
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* Opclass authors must consider the final cardinality of abbreviated keys
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* when devising an encoding scheme. It's possible for a strategy to work
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