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tiny fix on index selection #6265

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Aug 26, 2021
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tiny fix on index selection #6265

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4 changes: 2 additions & 2 deletions choose-index.md
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Expand Up @@ -39,15 +39,15 @@ TiDB uses the following heuristic pre-rules to select indexes:

+ Rule 1: If an index satisfies "unique index with full match + no need to retrieve rows from a table (which means that the plan generated by the index is the IndexReader operator)", TiDB directly selects this index.

+ Rule 2: If an index satisfies "unique index + the need to retrieve rows from a table (which means that the plan generated by the index is the IndexReader operator)", TiDB selects the index with the smallest number of rows to be retrieved from a table as a candidate index.
+ Rule 2: If an index satisfies "unique index with full match + the need to retrieve rows from a table (which means that the plan generated by the index is the IndexReader operator)", TiDB selects the index with the smallest number of rows to be retrieved from a table as a candidate index.

+ Rule 3: If an index satisfies "ordinary index + no need to retrieve rows from a table + the number of rows to be read is less than the value of a certain threshold", TiDB selects the index with the smallest number of rows to be read as a candidate index.

+ Rule 4: If only one candidate index is selected based on rule 2 and 3, select this candidate index. If two candidate indexes are respectively selected based on rule 2 and 3, select the index with the smaller number of rows to be read (the number of rows with index + the number of rows to be retrieved from a table).

The "index with full match" in the above rules means each indexed column has the equal condition. When executing the `EXPLAIN FORMAT = 'verbose' ...` statement, if the pre-rules match an index, TiDB outputs a NOTE-level warning indicating that the index matches the pre-rule.

In the following example, because the index `idx_b` meets the condition "unique index + the need to retrieve rows from a table" in rule 2, TiDB selects the index `idx_b` as the access path, and `SHOW WARNING` returns a note indicating that the index `idx_b` matches the pre-rule.
In the following example, because the index `idx_b` meets the condition "unique index with full match + the need to retrieve rows from a table" in rule 2, TiDB selects the index `idx_b` as the access path, and `SHOW WARNING` returns a note indicating that the index `idx_b` matches the pre-rule.

```sql
mysql> CREATE TABLE t(a INT PRIMARY KEY, b INT, c INT, UNIQUE INDEX idx_b(b));
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