Update system-variables.md (#18035) (#18110)

system-variables.md

@@ -992,6 +992,8 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - Use a bigger value in OLAP scenarios, and a smaller value in OLTP scenarios.
 - For OLAP scenarios, the maximum value should not 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 (determined by the size of the data to be scanned and the frequency of the scan) to avoid TiKV becoming out of memory (OOM).
+- For a simple query with only a `LIMIT` clause, if the `LIMIT` value is less than 100000, the scan operation pushed down to TiKV treats the value of this variable as `1` to enhance execution efficiency.
+- For the `SELECT MAX/MIN(col) FROM ...` query, if the `col` column has an index sorted in the same order required by the `MAX(col)` or `MIN(col)` function, TiDB will rewrite the query to `SELECT col FROM ... LIMIT 1` for processing, and the value of this variable will also be processed as `1`. For example, for `SELECT MIN(col) FROM ...`, if the `col` column has an ascending index, TiDB can quickly obtain the `MIN(col)` value by rewriting the query to `SELECT col FROM ... LIMIT 1` and directly reading the first row of the index.
 
 ### tidb_dml_batch_size