Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

tiny fix on index selection (#6265) #6266

Merged
merged 1 commit into from
Aug 26, 2021
Merged

tiny fix on index selection (#6265) #6266

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
4 changes: 2 additions & 2 deletions choose-index.md
View file
Open in desktop
Original file line number Diff line number Diff line change
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));
Expand Down