LIMIT pushdown to UNION ALL queries

[alexey-milovidov]

The first two queries below are slow due to missing optimization.
The last query is fast - this is how them are supposed to be optimized.

milovidov-desktop :) SELECT * FROM (SELECT * FROM numbers(1000000000) ORDER BY number UNION ALL SELECT * FROM numbers(1000000000) ORDER BY number) ORDER BY number LIMIT 10

SELECT *
FROM 
(
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
    UNION ALL
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
)
ORDER BY number ASC
LIMIT 10

Query id: ad5e3c17-6840-480a-ae12-27fe90d60e71

Cancelling query.
Ok.
Query was cancelled.

0 rows in set. Elapsed: 2.809 sec. Processed 203.20 million rows, 1.63 GB (72.34 million rows/s., 578.76 MB/s.) 

milovidov-desktop :) SELECT * FROM (SELECT * FROM numbers(1000000000) ORDER BY number UNION ALL SELECT * FROM numbers(1000000000) ORDER BY number) LIMIT 10

SELECT *
FROM 
(
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
    UNION ALL
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
)
LIMIT 10

Query id: 98d7340d-4efb-4c13-aa4a-2dddb3aa0602

→ Progress: 241.58 million rows, 1.93 GB (73.05 million rows/s., 584.37 MB/s.) ███████████████████▌                                                                                                                                               11%^Cancelling query.
Ok.
Query was cancelled.

0 rows in set. Elapsed: 3.509 sec. Processed 256.45 million rows, 2.05 GB (73.09 million rows/s., 584.70 MB/s.) 

milovidov-desktop :) SELECT * FROM (SELECT * FROM numbers(1000000000) ORDER BY number LIMIT 10 UNION ALL SELECT * FROM numbers(1000000000) ORDER BY number LIMIT 10) ORDER BY number LIMIT 10

SELECT *
FROM 
(
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
    LIMIT 10
    UNION ALL
    SELECT *
    FROM numbers(1000000000)
    ORDER BY number ASC
    LIMIT 10
)
ORDER BY number ASC
LIMIT 10

Query id: a550a49f-a792-4084-8294-e6e6304281ce

┌─number─┐
│      0 │
│      0 │
│      1 │
│      1 │
│      2 │
│      2 │
│      3 │
│      3 │
│      4 │
│      4 │
└────────┘

10 rows in set. Elapsed: 0.794 sec. Processed 2.00 billion rows, 16.00 GB (2.52 billion rows/s., 20.16 GB/s.)