There is no way to use unparallel projection executor

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Set tidb_projection_concurrency variable to 0 doesn't work, this variable is deprecated:

mysql> set @@tidb_projection_concurrency = 0;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                           |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1287 | 'tidb_projection_concurrency' is deprecated and will be removed in a future release. Please use tidb_executor_concurrency instead |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

Set variable tidb_executor_concurrency to 0 doesn't work, 0 is not a valid value for it:

mysql> set @@tidb_executor_concurrency = 0;
ERROR 1231 (42000): Variable 'tidb_executor_concurrency' can't be set to the value of '0'
mysql> set @@tidb_executor_concurrency = -1;
ERROR 1231 (42000): Variable 'tidb_executor_concurrency' can't be set to the value of '-1'
mysql> set @@tidb_executor_concurrency = -2;
ERROR 1231 (42000): Variable 'tidb_executor_concurrency' can't be set to the value of '-2'

Set variable tidb_executor_concurrency to 1 doesn't really work,

mysql> set @@tidb_executor_concurrency = 1;
Query OK, 0 rows affected (0.00 sec)

because the code requires the value less than or equal to 0:

tidb/executor/projection.go

Lines 185 to 187 in efc774a

	func (e *ProjectionExec) isUnparallelExec() bool {
	return e.numWorkers <= 0
	}

So, when we set tidb_executor_concurrency to 1, we actually spawn one worker and the main goroutine is waiting for the worker to finish the job.

This code branch is never covered

tidb/executor/projection.go

Line 179 in efc774a

return e.unParallelExecute(ctx, req)

2. What did you expect to see? (Required)

I want to make sure projection runs non-concurrently ...

3. What did you see instead (Required)

There is no way to do that.

4. What is your TiDB version? (Required)

commit efc774a (HEAD -> master, origin/master, origin/HEAD)
Author: ZhuoZhi 517770911@qq.com
Date: Wed Jun 16 15:12:37 2021 +0800

planner: check filter condition in func convertToPartialTableScan (#25294)