xiedeyantu commented on code in PR #4636:
URL: https://github.com/apache/calcite/pull/4636#discussion_r2543849867
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core/src/main/java/org/apache/calcite/rel/rules/AggregateExpandDistinctAggregatesRule.java:
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@@ -425,39 +426,118 @@ private static RelBuilder
convertSingletonDistinct(RelBuilder relBuilder,
return relBuilder;
}
+ /**
+ * Rewrite aggregates that use GROUPING SETS. The following SQL/plan example
+ * serves as the concrete blueprint, starting from the original statement and
+ * plan-before outputs and then rebuilding the plan-after tree from the
bottom
+ * (line 7) back to the top (line 1):
+ *
+ * <p>Original SQL:
+ * <pre>{@code
+ * SELECT deptno, COUNT(DISTINCT sal)
+ * FROM emp
+ * GROUP BY ROLLUP(deptno)
+ * }</pre>
+ *
+ * <p>Plan before rewrite:
+ * <pre>{@code
+ * LogicalAggregate(group=[{0}], groups=[[{0}, {}]], EXPR$1=[COUNT(DISTINCT
$1)])
+ * LogicalProject(DEPTNO=[$7], SAL=[$5])
+ * LogicalTableScan(table=[[CATALOG, SALES, EMP]])
+ * }</pre>
+ *
+ * <p>Plan after rewrite (lines referenced below):
+ * <pre>{@code
+ * 1 LogicalProject(DEPTNO=[$0],
+ * EXPR$1=[CAST(CASE(=($5, 0), $1, =($5, 1), $2,
+ * null:BIGINT)):BIGINT NOT NULL])
+ * 2 LogicalFilter(condition=[OR(
+ * AND(=($5, 0), >($3, 0)), =($5, 1))])
+ * 3 LogicalAggregate(group=[{0}], groups=[[{0}, {}]],
+ * EXPR$1_g0=[COUNT($1) FILTER $2],
+ * EXPR$1_g1=[COUNT($1) FILTER $4],
+ * $g_present_0=[COUNT() FILTER $3],
+ * $g_present_1=[COUNT() FILTER $5],
+ * $g_final=[GROUPING($0)])
+ * 4 LogicalProject(DEPTNO=[$0], SAL=[$1],
Review Comment:
Indeed, I have considered this issue and also attempted to see if it's
possible to remove unnecessary groupings to eliminate the second-to-last Filter
layer. However, to accommodate various scenarios, I haven't yet found a good
solution. Perhaps we can find ways to optimize this later. When I submitted
this PR, I also felt that the resulting plan was somewhat complex...
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