PS-10232 [8.0]: Fix performance regression by limiting rows_in_buffer to query

`LIMIT`

A follow up fix (2969af5) for "Bug#36775910: Record buffer not set in index
range scans" caused a performance regression by failing to respect the handler's
recommendation to cap on number of rows in the record buffer (at the time of
writing hard-coded to 100 for InnoDB). This caused a performance regression
which is most pronounced when using a small LIMIT.

Fixed by re-introducing the cap.

Future work: It would likely be beneficial to communicate the LIMIT value to the
executor so that it could reduce the buffer size accordingly - or not allocate a
buffer at all in case of LIMIT 1.

Author: Martin Hansson

sql/sql_executor.cc

@@ -739,6 +739,9 @@ bool set_record_buffer(TABLE *table, double expected_rows_to_fetch) {
       const ha_rows local_max_rows = (MAX_RECORD_BUFFER_SIZE / record_size);
       rows_in_buffer = std::clamp(rows_in_buffer, min_rows, local_max_rows);
     }
+
+    // Cap the number of rows according to handler's wish.
+    rows_in_buffer = std::min(rows_in_buffer, max_rows);
   }
 
   // After adjustments made above, we still need a minimum of 2 rows to