MDEV-39773 Prefix index on VARCHAR PRIMARY KEY column causes equality lookup to return 0 rows (wrong result) when optimizer chooses const access via the prefix index - #5709
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… lookup to return 0 rows (wrong result) when optimizer chooses const access via the prefix index An equality lookup through a VARCHAR prefix index can miss an existing row when the optimizer treats that index as a unique, single-row access path. Extended-key construction considers a primary-key field covered by a secondary-key part using only fieldnr. A prefix of the same field is not equivalent to its full primary-key part, but can consequently receive the uniqueness properties of the full key. Require both the field number and key-part length to match before treating a secondary-key part as covering a primary-key part. Keep the full primary-key part available when the secondary index contains only a prefix, preventing an invalid HA_EXT_NOSAME classification. The regression inserts -1 and -2 into an InnoDB table with a VARCHAR primary key, adds a one-character prefix index, and requires a forced prefix-index lookup for -2 to return that row. Bug report: https://jira.mariadb.org/browse/MDEV-39773
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An equality lookup through a VARCHAR prefix index can miss an existing row when the optimizer treats that index as a unique, single-row access path.
Bug report: https://jira.mariadb.org/browse/MDEV-39773
Root cause
Extended-key construction considers a primary-key field covered by a secondary-key part using only fieldnr. A prefix of the same field is not equivalent to its full primary-key part, but can consequently receive the uniqueness properties of the full key.
Changes
Regression coverage
The regression inserts -1 and -2 into an InnoDB table with a VARCHAR primary key, adds a one-character prefix index, and requires a forced prefix-index lookup for -2 to return that row.
mysql-test/main/prefix_index_primary_key.testmysql-test/main/prefix_index_primary_key.resultValidation
On
11.4atd10e5d726799b1cd57cc866f40aad68c720803da:main.prefix_index_primary_keyfailed on the unchanged target branch and passed with this fix.main.select.