Pushdown LIMIT to UNION ALL query

[ucasfl]

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Changelog category (leave one):

• Performance Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Pushdown LIMIT to UNION ALL query. closes #23239.

Detailed description / Documentation draft:

:) explain syntax SELECT * FROM (SELECT * FROM numbers(100) ORDER BY number union all select * from numbers(100) order by number ) 
ORDER BY number LIMIT 10                                                                                                                          
                                                                                                                                                  
EXPLAIN SYNTAX                                                                                                                                    
SELECT *                                                                                                                                          
FROM                                                                                                                                              
(                                                                                                                                                 
    SELECT *                                                                                                                                      
    FROM numbers(100)
    ORDER BY number ASC
    UNION ALL
    SELECT *
    FROM numbers(100)
    ORDER BY number ASC
)
ORDER BY number ASC
LIMIT 10

Query id: d470df8c-fd26-40ce-b27e-76633c8be785

┌─explain─────────────────┐
│ SELECT number           │
│ FROM                    │
│ (                       │
│     SELECT number       │
│     FROM numbers(100)   │
│     ORDER BY number ASC │
│     LIMIT 10            │
│     UNION ALL           │
│     SELECT number       │
│     FROM numbers(100)   │
│     ORDER BY number ASC │
│     LIMIT 10            │
│ )                       │
│ ORDER BY number ASC     │
│ LIMIT 10                │
└─────────────────────────┘

15 rows in set. Elapsed: 0.015 sec.

[ucasfl]

Change in here just because an unrelated format issue.

[anotherbugmaster]

@ucasfl have you decided to close this because there are better ways to solve the issue or for other reasons?

[alexey-milovidov]

@ucasfl, why closed?