"Union all" query seems to spend lot of time in "statistics" per profiling

MySQL & MariaDB Percona Server for MySQL 8.0
1

Hi Folks,

Per the profiling, our “union all” queries seem to spend lot of time in “statistics”.

Copy/pasted that line.

| Status | Duration | Source_function | Source_file | Source_line |
| statistics | 0.000476 | optimize | sql_optimizer.cc | 530

The selects inside union all don’t have any joins, aggregations or subqueries.
The tables have only primary index. No secondary indexes. The PK has 4 columns. All selects have equality predicate on at least 1 column of PK.

Any idea why “union all” spending lot of time in “statistics”? Any config to reduce time spent in it.

Thanks,
Vamsi

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2

Also, our tables are RocksDB tables. In my previous post, I just showed one line from profiling for “statistics”. We see multiple such lines (as many as selects inside the union all).

Thanks,
Vamsi

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