[Experiment] what if reorder compare of columns in Merge

[UnamedRus]

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• Performance Improvement

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Start to compare rows from most likely unequal columns first.

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• Documentation is written (mandatory for new features)

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[robot-ch-test-poll4]

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[alexey-milovidov]

Did it help?

[UnamedRus]

Did it help?

Doesn't seems so.
But may be, i was bad on making good test for that.

[alexey-milovidov]

We need at least one proof, even synthetic will be ok.

[UnamedRus]

Test with LC(String)

New 
SELECT count()
FROM test_reorder_columns_in_merge
FINAL
SETTINGS max_threads = 1

   ┌──count()─┐
1. │ 60000000 │ -- 60.00 million
   └──────────┘

1 row in set. Elapsed: 3.120 sec. Processed 59.91 million rows, 479.32 MB (19.20 million rows/s., 153.63 MB/s.)
Peak memory usage: 4.06 MiB.

Old
SELECT count()
FROM test_reorder_columns_in_merge
FINAL
SETTINGS max_threads = 1

   ┌──count()─┐
1. │ 60000000 │ -- 60.00 million
   └──────────┘

1 row in set. Elapsed: 4.134 sec. Processed 59.91 million rows, 479.32 MB (14.49 million rows/s., 115.94 MB/s.)
Peak memory usage: 4.06 MiB.

[alexey-milovidov]

I'm afraid of this division - change to something like if and reset to zero.

[UnamedRus]

Not sure that simple reset to 0 will work

But something like, could.
But not sure that it's better. (and size_t cannot be negative, so different type)

if ((offset + i) == size)
   offset -= size;

auto col_number = (offset + i);