maropu commented on a change in pull request #27058: [SPARK-30276][SQL] Support
Filter expression allows simultaneous use of DISTINCT
URL: https://github.com/apache/spark/pull/27058#discussion_r368805239
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/RewriteDistinctAggregates.scala
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@@ -118,7 +118,66 @@ import org.apache.spark.sql.types.IntegerType
* LocalTableScan [...]
* }}}
*
- * The rule does the following things here:
+ * Third example: single distinct aggregate function with filter clauses (in
sql):
+ * {{{
+ * SELECT
+ * COUNT(DISTINCT cat1) FILTER (WHERE id > 1) as cat1_cnt1,
+ * COUNT(DISTINCT cat1) as cat1_cnt2,
+ * SUM(value) AS total
+ * FROM
+ * data
+ * GROUP BY
+ * key
+ * }}}
+ *
+ * This translates to the following (pseudo) logical plan:
+ * {{{
+ * Aggregate(
+ * key = ['key]
+ * functions = [COUNT(DISTINCT 'cat1) with FILTER('id > 1),
+ * COUNT(DISTINCT 'cat1),
+ * sum('value)]
+ * output = ['key, 'cat1_cnt1, 'cat1_cnt2, 'total])
+ * LocalTableScan [...]
+ * }}}
+ *
+ * This rule rewrites this logical plan to the following (pseudo) logical plan:
+ * {{{
+ * Aggregate(
+ * key = ['key]
+ * functions = [count(if (('gid = 1)) '_gen_distinct_1 else null),
+ * count(if (('gid = 2)) '_gen_distinct_2 else null),
+ * first(if (('gid = 0)) 'total else null) ignore nulls]
+ * output = ['key, 'cat1_cnt, 'cat1_cnt2, 'total])
+ * Aggregate(
+ * key = ['key, '_gen_distinct_1, '_gen_distinct_2, 'gid]
+ * functions = [sum('value)]
+ * output = ['key, '_gen_distinct_1, '_gen_distinct_2, 'gid, 'total])
+ * Expand(
+ * projections = [('key, null, null, 0, 'value),
+ * ('key, '_gen_distinct_1, null, 1, null),
+ * ('key, null, '_gen_distinct_2, 2, null)]
+ * output = ['key, '_gen_distinct_1, '_gen_distinct_2, 'gid, 'value])
+ * Expand(
+ * projections = [('key, if ('id > 1) 'cat1 else null, 'cat1,
cast('value as bigint))]
+ * output = ['key, '_gen_distinct_1, '_gen_distinct_2, 'value])
+ * LocalTableScan [...]
+ * }}}
+ *
+ * The rule serves two purposes:
+ * 1. Expand distinct aggregates which exists filter clause.
Review comment:
Also, to simplify the structure, how about this format?;
```
* The rule consists of the two phases as follows;
*
* In the first phase, expands distinct aggregates which exists filter
clause:
* 1. Guaranteed to compute filter clause locally.
* ...
*
* In the second phase, rewrite when aggregate exists at least two distinct
aggregates:
* 1. Expand the data. There are three aggregation groups in this query:
* i. the non-distinct group;
* ii. the distinct 'cat1 group;
* iii. the distinct 'cat2 group.
* An expand operator is inserted to expand the child data for each
group. The expand will null
* out all unused columns for the given group; this must be done in order
to ensure correctness
* later on. Groups can by identified by a group id (gid) column added by
the expand operator.
* ...
*
```
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