Update the description of tidb_distsql_scan_concurrency (#4401)

* update the description of tidb_distsql_scan_concurrency * Apply suggestions from code review Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com> Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>
pingcap · Dec 17, 2020 · e519708 · e519708
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diff --git a/system-variables.md b/system-variables.md
@@ -275,6 +275,7 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - This variable is used to set the concurrency of the `scan` operation.
 - Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
 - For OLAP scenarios, the maximum value cannot exceed the number of CPU cores of all the TiKV nodes.
+- If a table has a lot of partitions, you can reduce the variable value appropriately to avoid TiKV becoming out of memory (OOM).
 
 ### tidb_dml_batch_size