[jira] [Created] (CALCITE-4491) Aggregation of window function produces invalid SQL for PostgreSQL

["Dominik Labuda (Jira)" <ji...@apache.org>]

Dominik Labuda created CALCITE-4491:
---------------------------------------

             Summary: Aggregation of window function produces invalid SQL for PostgreSQL
                 Key: CALCITE-4491
                 URL: https://issues.apache.org/jira/browse/CALCITE-4491
             Project: Calcite
          Issue Type: Bug
          Components: core
    Affects Versions: 1.26.0
            Reporter: Dominik Labuda


h2. Issue

We tested the following behavior against PostgreSQL database, however more of the supported dialects may be affected.

When aggregating the results of a window function an invalid SQL is generated. I was able to replicate the behavior in tests with:
{code:java}
@Test void testAggregatingWindowFunction() {
    final RelBuilder builder = relBuilder();
    final RelNode root = builder
        .scan("EMP")
        .project(
            builder.alias(
                builder.getRexBuilder().makeOver(
                    builder.getTypeFactory().createSqlType(SqlTypeName.INTEGER),
                    SqlStdOperatorTable.RANK,
                    new ArrayList<>(),
                    new ArrayList<>(),
                    ImmutableList.of(new RexFieldCollation(builder.field("SAL"), ImmutableSet.of())),
                    RexWindowBounds.UNBOUNDED_PRECEDING,
                    RexWindowBounds.UNBOUNDED_FOLLOWING,
                    true,
                    true,
                    false,
                    false,
                    false
                ),
                "rank"
            )
        )
        .as("tmp")
        .aggregate(
            builder.groupKey(),
            builder.count(
                true,
                "cnt",
                builder.field("tmp", "rank")
            )
        )
        .build();
    final String expectedSql = "SELECT COUNT(DISTINCT \"rank\") AS \"cnt\"\n" +
        "FROM (SELECT RANK() OVER (ORDER BY \"SAL\") AS \"rank\"\n" +
        "FROM \"scott\".\"EMP\") AS \"t\"";
    assertThat(
        toSql(root, PostgresqlSqlDialect.DEFAULT),
        isLinux(expectedSql)
    );
  }
{code}
The code above fails, since it produces this SQL instead of the expected one, which cannot be executed on PGSQL:
{noformat}
SELECT COUNT(DISTINCT RANK() OVER (ORDER BY "SAL")) AS "cnt"
FROM "scott"."EMP"
{noformat}
In that case I am getting this kinds of errors from DB:
{noformat}
ERROR: aggregate function calls cannot contain window function calls
{noformat}

h2. Suggested solution

Since _SqlDialect_ already contains support for determining whether the databases support nested aggregations via _public boolean supportsNestedAggregations()_ we could either add another method like _public boolean supportsNestedWindows()_ or reuse the existing method for the window purposes (which seems non-transparent). Then we will be able to handle the behavior in _SqlImplementor.needNewSubQuery()_ similarly as it already does with the aggregations.

Please let me know if you agree and I will be more than happy to provide you with PR to review, thanks!



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