suibianwanwank commented on code in PR #4181:
URL: https://github.com/apache/calcite/pull/4181#discussion_r1950153334
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core/src/main/java/org/apache/calcite/sql2rel/RelDecorrelator.java:
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@@ -795,6 +801,122 @@ private static void shiftMapping(Map<Integer, Integer>
mapping, int startIndex,
return null;
}
+ private @Nullable Frame decorrelateFetchOneSort(Sort sort, final Frame
frame) {
+ final Map<Integer, Integer> mapOldToNewOutputs = new HashMap<>();
+ final NavigableMap<CorDef, Integer> corDefOutputs = new TreeMap<>();
+ //
+ // Rewrite logic:
+ //
+ // If sorted with no OFFSET and FETCH = 1, and only one collation field,
+ // rewrite the Sort as Aggregate using MIN/MAX function.
+ // Example:
+ // Sort(sort0=[$0], dir0=[ASC], fetch=[1])
+ // input
+ // Rewrite to:
+ // Aggregate(group=(corVar), agg=[min($0))
+ //
+ // Note: MIN/MAX is not strictly equivalent to LIMIT 1. When the input has
0 rows,
+ // MIN/MAX returns NULL, while LIMIT 1 returns 0 rows.
+ // However, in the decorrelate, we add correlated variables to the group
list
+ // to ensure equivalence when Correlate is transformed to Join. When the
group list
+ // is non-empty, MIN/MAX will also return 0 rows if input with 0
rows.(This behavior
+ // also leads to the issue described in [CALCITE-5743]) So in this case,
the transformation
+ // is legal.
+ if (sort.getCollation().getFieldCollations().size() == 1
+ && sort.getRowType().getFieldCount() == 1
+ && !frame.corDefOutputs.isEmpty()) {
+ RelFieldCollation collation =
Util.first(sort.getCollation().getFieldCollations());
+
+ SqlAggFunction aggFunction;
+ switch (collation.getDirection()) {
+ case ASCENDING:
+ case STRICTLY_ASCENDING:
+ aggFunction = SqlStdOperatorTable.MIN;
+ break;
+ case DESCENDING:
+ case STRICTLY_DESCENDING:
+ aggFunction = SqlStdOperatorTable.MAX;
+ break;
+ default:
+ return null;
+ }
+
+ final int newIdx =
requireNonNull(frame.oldToNewOutputs.get(collation.getFieldIndex()));
+ RelBuilder.AggCall aggCall = relBuilder.push(frame.r)
Review Comment:
For MIN/MAX, null values are ignored in the databases I know (I didn't find
a corresponding description in the sql standard). So I think both asc and desc
should be null last, i.e.return null only if all values are null.
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