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Posted to reviews@spark.apache.org by GitBox <gi...@apache.org> on 2022/03/25 12:01:52 UTC
[GitHub] [spark] beliefer opened a new pull request #35975: [SPARK-28330][SQL] Support ANSI SQL: result offset clause in query expression
beliefer opened a new pull request #35975:
URL: https://github.com/apache/spark/pull/35975
### What changes were proposed in this pull request?
This is a ANSI SQL and feature id is `F861`
```
<query expression> ::=
[ <with clause> ] <query expression body>
[ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ]
<result offset clause> ::=
OFFSET <offset row count> { ROW | ROWS }
```
For example:
```
SELECT customer_name, customer_gender FROM customer_dimension
WHERE occupation='Dancer' AND customer_city = 'San Francisco' ORDER BY customer_name;
customer_name | customer_gender
----------------------+-----------------
Amy X. Lang | Female
Anna H. Li | Female
Brian O. Weaver | Male
Craig O. Pavlov | Male
Doug Z. Goldberg | Male
Harold S. Jones | Male
Jack E. Perkins | Male
Joseph W. Overstreet | Male
Kevin . Campbell | Male
Raja Y. Wilson | Male
Samantha O. Brown | Female
Steve H. Gauthier | Male
William . Nielson | Male
William Z. Roy | Male
(14 rows)
SELECT customer_name, customer_gender FROM customer_dimension
WHERE occupation='Dancer' AND customer_city = 'San Francisco' ORDER BY customer_name OFFSET 8;
customer_name | customer_gender
-------------------+-----------------
Kevin . Campbell | Male
Raja Y. Wilson | Male
Samantha O. Brown | Female
Steve H. Gauthier | Male
William . Nielson | Male
William Z. Roy | Male
(6 rows)
```
There are some mainstream database support the syntax.
**Druid**
https://druid.apache.org/docs/latest/querying/sql.html#offset
**Kylin**
http://kylin.apache.org/docs/tutorial/sql_reference.html#QUERYSYNTAX
**Exasol**
https://docs.exasol.com/sql/select.htm
**Greenplum**
http://docs.greenplum.org/6-8/ref_guide/sql_commands/SELECT.html
**MySQL**
https://dev.mysql.com/doc/refman/5.6/en/select.html
**Monetdb**
https://www.monetdb.org/Documentation/SQLreference/SQLSyntaxOverview#SELECT
**PostgreSQL**
https://www.postgresql.org/docs/11/queries-limit.html
**Sqlite**
https://www.sqlite.org/lang_select.html
**Vertica**
https://www.vertica.com/docs/9.2.x/HTML/Content/Authoring/SQLReferenceManual/Statements/SELECT/OFFSETClause.htm?zoom_highlight=offset
The description for design:
**1**. Consider `OFFSET` as the special case of `LIMIT`. For example:
`SELECT * FROM a limit 10;` similar to `SELECT * FROM a limit 10 offset 0;`
`SELECT * FROM a offset 10;` similar to `SELECT * FROM a limit -1 offset 10;`
**2**. Because the current implement of `LIMIT` has good performance. For example:
`SELECT * FROM a limit 10;` parsed to the logic plan as below:
```
GlobalLimit (limit = 10)
|--LocalLimit (limit = 10)
```
and then the physical plan as below:
```
GlobalLimitExec (limit = 10) // Take the first 10 rows globally
|--LocalLimitExec (limit = 10) // Take the first 10 rows locally
```
This operator reduce massive shuffle and has good performance.
Sometimes, the logic plan transformed to the physical plan as:
```
CollectLimitExec (limit = 10) // Take the first 10 rows globally
```
If the SQL contains order by, such as `SELECT * FROM a order by c limit 10;`.
This SQL will be transformed to the physical plan as below:
```
TakeOrderedAndProjectExec (limit = 10) // Take the first 10 rows after sort globally
```
Based on this situation, this PR produces the following operations. For example:
`SELECT * FROM a limit 10 offset 10;` parsed to the logic plan as below:
```
GlobalLimit (limit = 10)
|--LocalLimit (limit = 10)
|--Offset (offset = 10)
```
After optimization, the above logic plan will be transformed to:
```
GlobalLimitAndOffset (limit = 10, offset = 10) // Limit clause accompanied by offset clause
|--LocalLimit (limit = 20) // 10 + offset = 20
```
and then the physical plan as below:
```
GlobalLimitAndOffsetExec (limit = 10, offset = 10) // Skip the first 10 rows and take the next 10 rows globally
|--LocalLimitExec (limit = 20) // Take the first 20(limit + offset) rows locally
```
Sometimes, the logic plan transformed to the physical plan as:
```
CollectLimitExec (limit = 10, offset = 10) // Skip the first 10 rows and take the next 10 rows globally
```
If the SQL contains order by, such as `SELECT * FROM a order by c limit 10 offset 10;`.
This SQL will be transformed to the physical plan as below:
```
TakeOrderedAndProjectExec (limit = 10, offset 10) // Skip the first 10 rows and take the next 10 rows after sort globally
```
**3**.In addition to the above, there is a special case that is only offset but no limit. For example:
`SELECT * FROM a offset 10;` parsed to the logic plan as below:
```
Offset (offset = 10) // Only offset clause
```
If offset is very large, will generate a lot of overhead. So this PR will refuse use offset clause without limit clause, although we can parse, transform and execute it.
A balanced idea is add a configuration item `spark.sql.forceUsingOffsetWithoutLimit` to force running query when user knows the offset is small enough. The default value of `spark.sql.forceUsingOffsetWithoutLimit` is false. This PR just came up with the idea so that it could be implemented at a better time in the future.
Note: The origin PR to support this feature is https://github.com/apache/spark/pull/25416.
Because the origin PR too old, there exists massive conflict which is hard to resolve. So I open this new PR to support this feature.
### Why are the changes needed?
new feature
### Does this PR introduce any user-facing change?
'No'
### How was this patch tested?
Exists and new UT
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[GitHub] [spark] beliefer commented on pull request #35975: [SPARK-28330][SQL] Support ANSI SQL: result offset clause in query expression
Posted by GitBox <gi...@apache.org>.
beliefer commented on pull request #35975:
URL: https://github.com/apache/spark/pull/35975#issuecomment-1079639707
ping @cloud-fan
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