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From "Takeshi Yamamuro (Jira)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-29699) Different answers in nested aggregates with window functions
Date Mon, 23 Dec 2019 06:10:00 GMT

    [ https://issues.apache.org/jira/browse/SPARK-29699?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17002122#comment-17002122
] 

Takeshi Yamamuro commented on SPARK-29699:
------------------------------------------

A root cause is the NULL order difference in order-by clauses; in Spark, NULLS FIRST is the
default for an ascending order, but in PostgreSQL/Oracle, NULLS LAST is the default for that.
So, if we explicitly set the order in the example query above, we can get the same answer
with PostgreSQL;
{code:java}
sql("""select a, b, sum(c), sum(sum(c)) over (order by a asc nulls last, b asc nulls last)
as rsum
  from gstest2 group by rollup (a,b) order by rsum asc nulls last, a asc nulls last, b asc
nulls last""").show()
+----+----+------+----+                                                         
|   a|   b|sum(c)|rsum|
+----+----+------+----+
|   1|   1|     8|   8|
|   1|   2|     2|  10|
|   1|null|    10|  20|
|   2|   2|     2|  22|
|   2|null|     2|  24|
|null|null|    12|  36|
+----+----+------+----+
{code}
Currently, it seems we follow the MySQL/SQL Server behaviour and they have NULLS FIRST by
default.
 Any historical reason for our default NULL order? cc: [~smilegator] [~cloud_fan] [~viirya]

Changing the default behaivour in Spark has some impacts on test output in SQLQueryTestSuite:
 [https://github.com/apache/spark/compare/master...maropu:NullLastByDefault]

 

References:
 Cited from the PostgreSQL doc: [https://www.postgresql.org/docs/current/queries-order.html]
{code:java}
By default, null values sort as if larger than any non-null value; that is, NULLS FIRST is
the default for DESC order, and NULLS LAST otherwise.
{code}
Cited from the OracleDB doc: [https://docs.oracle.com/database/121/SQLRF/statements_10002.htm#SQLRF01702]
{code:java}
NULLS LAST is the default for ascending order, and NULLS FIRST is the default for descending
order.
{code}
Cited from the SQL server: [https://docs.microsoft.com/en-us/sql/t-sql/queries/select-order-by-clause-transact-sql?redirectedfrom=MSDN&view=sql-server-ver15]
{code:java}
 ASC is the default sort order. Null values are treated as the lowest possible values.
{code}
Cited from the MySQL: [https://dev.mysql.com/doc/refman/5.7/en/working-with-null.html]
{code:java}
When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last
if you do ORDER BY ... DESC.
{code}

> Different answers in nested aggregates with window functions
> ------------------------------------------------------------
>
>                 Key: SPARK-29699
>                 URL: https://issues.apache.org/jira/browse/SPARK-29699
>             Project: Spark
>          Issue Type: Sub-task
>          Components: SQL
>    Affects Versions: 3.0.0
>            Reporter: Takeshi Yamamuro
>            Priority: Major
>              Labels: correctness
>
> A nested aggregate below with a window function seems to have different answers in the
`rsum` column  between PgSQL and Spark;
> {code:java}
> postgres=# create table gstest2 (a integer, b integer, c integer, d integer, e integer,
f integer, g integer, h integer);
> postgres=# insert into gstest2 values
> postgres-#   (1, 1, 1, 1, 1, 1, 1, 1),
> postgres-#   (1, 1, 1, 1, 1, 1, 1, 2),
> postgres-#   (1, 1, 1, 1, 1, 1, 2, 2),
> postgres-#   (1, 1, 1, 1, 1, 2, 2, 2),
> postgres-#   (1, 1, 1, 1, 2, 2, 2, 2),
> postgres-#   (1, 1, 1, 2, 2, 2, 2, 2),
> postgres-#   (1, 1, 2, 2, 2, 2, 2, 2),
> postgres-#   (1, 2, 2, 2, 2, 2, 2, 2),
> postgres-#   (2, 2, 2, 2, 2, 2, 2, 2);
> INSERT 0 9
> postgres=# 
> postgres=# select a, b, sum(c), sum(sum(c)) over (order by a,b) as rsum
> postgres-#   from gstest2 group by rollup (a,b) order by rsum, a, b;
>  a | b | sum | rsum 
> ---+---+-----+------
>  1 | 1 |   8 |    8
>  1 | 2 |   2 |   10
>  1 |   |  10 |   20
>  2 | 2 |   2 |   22
>  2 |   |   2 |   24
>    |   |  12 |   36
> (6 rows)
> {code}
> {code:java}
> scala> sql("""
>      | select a, b, sum(c), sum(sum(c)) over (order by a,b) as rsum
>      |   from gstest2 group by rollup (a,b) order by rsum, a, b
>      | """).show()
> +----+----+------+----+                                                         
> |   a|   b|sum(c)|rsum|
> +----+----+------+----+
> |null|null|    12|  12|
> |   1|null|    10|  22|
> |   1|   1|     8|  30|
> |   1|   2|     2|  32|
> |   2|null|     2|  34|
> |   2|   2|     2|  36|
> +----+----+------+----+
> {code}



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