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Comment improvements for abbreviated keys.

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Peter Geoghegan and Robert Haas
This commit is contained in:
Robert Haas
2015-12-22 13:57:18 -05:00
parent ccd8f97922
commit 0ba3f3bc65
2 changed files with 31 additions and 23 deletions
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17
src/backend/utils/sort/tuplesort.c
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@@ -930,7 +930,14 @@ tuplesort_begin_datum(Oid datumType, Oid sortOperator, Oid sortCollation,
state->sortKeys->ssup_collation = sortCollation;
state->sortKeys->ssup_nulls_first = nullsFirstFlag;
/* abbreviation is possible here only for by-reference types */
/*
* Abbreviation is possible here only for by-reference types. In theory,
* a pass-by-value datatype could have an abbreviated form that is cheaper
* to compare. In a tuple sort, we could support that, because we can
* always extract the original datum from the tuple is needed. Here, we
* can't, because a datum sort only stores a single copy of the datum;
* the "tuple" field of each sortTuple is NULL.
*/
state->sortKeys->abbreviate = !typbyval;
PrepareSortSupportFromOrderingOp(sortOperator, state->sortKeys);
@@ -1271,7 +1278,7 @@ tuplesort_putindextuplevalues(Tuplesortstate *state, Relation rel,
* Store ordinary Datum representation, or NULL value. If there is a
* converter it won't expect NULL values, and cost model is not
* required to account for NULL, so in that case we avoid calling
* converter and just set datum1 to "void" representation (to be
* converter and just set datum1 to zeroed representation (to be
* consistent).
*/
stup.datum1 = original;
@@ -3155,7 +3162,7 @@ copytup_heap(Tuplesortstate *state, SortTuple *stup, void *tup)
* Store ordinary Datum representation, or NULL value. If there is a
* converter it won't expect NULL values, and cost model is not
* required to account for NULL, so in that case we avoid calling
* converter and just set datum1 to "void" representation (to be
* converter and just set datum1 to zeroed representation (to be
* consistent).
*/
stup->datum1 = original;
@@ -3397,7 +3404,7 @@ copytup_cluster(Tuplesortstate *state, SortTuple *stup, void *tup)
* Store ordinary Datum representation, or NULL value. If there is a
* converter it won't expect NULL values, and cost model is not
* required to account for NULL, so in that case we avoid calling
* converter and just set datum1 to "void" representation (to be
* converter and just set datum1 to zeroed representation (to be
* consistent).
*/
stup->datum1 = original;
@@ -3701,7 +3708,7 @@ copytup_index(Tuplesortstate *state, SortTuple *stup, void *tup)
* Store ordinary Datum representation, or NULL value. If there is a
* converter it won't expect NULL values, and cost model is not
* required to account for NULL, so in that case we avoid calling
* converter and just set datum1 to "void" representation (to be
* converter and just set datum1 to zeroed representation (to be
* consistent).
*/
stup->datum1 = original;