fix improper expression

choose-index.md

@@ -29,19 +29,19 @@ Before introducing index selection, it is important to understand the ways TiDB
 
 ## Index selection rules
 
-TiDB selects indexes based on rules or cost. the based rules include pre rules and Skyline-pruning. When selecting an index, TiDB tries the pre rule first. If the existing index satisfies a pre rule, TiDB will directly select the index. Otherwise, TiDB will use Skyline-Pruning to exclude unqualified indexes, and then based on the cost estimation of each operator for accessing tables, select the index with the lowest cost.
+TiDB selects indexes based on rules or cost. the based rules include pre rules and Skyline-pruning. When selecting an index, TiDB tries the pre rule first. If an index satisfies a pre rule, TiDB will directly select the index. Otherwise, TiDB will use Skyline-Pruning to exclude unqualified indexes, and then based on the cost estimation of each operator for accessing tables, select the index with the lowest cost.
 
 ### Selection based on rules
 
 #### Pre rules
 
 TiDB uses the following heuristic pre rules to select index:
 
-+ rule 1: If the existing index satisfies "unique indexes with full match + no need to retrieve rows from a table (which means the plan generated by the index is IndexReader operator)", directly select this index.
++ rule 1: If an index satisfies "unique indexes with full match + no need to retrieve rows from a table (which means the plan generated by the index is IndexReader operator)", directly select this index.
 
-+ rule 2: If the existing index satisfies "unique indexes + need to retrieve rows from a table (which means the plan generated by the index is IndexReader operator)", select the index with the smallest number of retrieve rows from a table as the candidate index.
++ rule 2: If an index satisfies "unique indexes + need to retrieve rows from a table (which means the plan generated by the index is IndexReader operator)", select the index with the smallest number of retrieve rows from a table as the candidate index.
 
-+ rule 3: If the existing index satisfies "common indexes + no need to retrieve rows from a table + the number of rows to be read is less than the value of a certain threshold", select the index with the smallest number of rows to be read as the candidate index.
++ rule 3: If an index satisfies "common indexes + no need to retrieve rows from a table + the number of rows to be read is less than the value of a certain threshold", select the index with the smallest number of rows to be read as the candidate index.
 
 + rule 4: If only one candidate index is selected based on rule 2 and 3, select this index. If two candidate index is separately selected based on rule 2 and 3, select the index with the smallest number of rows to be read (the number of rows with index + the number of rows to be retrieved from a table).