identify-index-issues.md

title	excerpt	products	keywords
Identify and resolve issues with indexes in Managed Service for TimescaleDB	Identify and resolve issues with PostgreSQL indexes in Managed Service for TimescaleDB	mst	index REINDEX

Identify and repair issues with PostgreSQL indexes with REINDEX

PostgreSQL indexes can be corrupted for a variety of reasons, including software bugs, hardware failures, or unexpected duplicated data. REINDEX allows you to rebuild the index in such situations.

Rebuild non-unique indexes

You can rebuild corrupted indexes that do not have UNIQUE in their definition. You can run the REINDEX command for all indexes of a table (REINDEX TABLE), and for all indexes in the entire database (REINDEX DATABASE). For more information on the REINDEX command, see the PostgreSQL documentation.

This command creates a new index that replaces the old one:

REINDEX INDEX <index-name>;

When you use `REINDEX`, the tables are locked and you may not be able to use the database, until the operation is complete.

In some cases, you might need to manually build a second index concurrently with the old index, and then remove the old index:

CREATE INDEX CONCURRENTLY test_index_new ON table_a (...);
DROP INDEX CONCURRENTLY test_index_old;
ALTER INDEX test_index_new RENAME TO test_index;

Rebuild unique indexes

A UNIQUE index works on one or more columns where the combination is unique in the table. When the index is corrupted or disabled, duplicated physical rows appear in the table, breaking the uniqueness constraint of the index. When you try to rebuild an index that is not unique, the REINDEX command fails. To resolve this issue, first remove the duplicate rows from the table and then rebuild the index.

Identify conflicting duplicated rows

To identify conflicting duplicate rows, you need to run a query that counts the number of rows for each combination of columns included in the index definition.

For example, this route table has a unique_route_index index defining unique rows based on the combination of the source and destination columns:

CREATE TABLE route(
    source TEXT,
    destination TEXT,
    description TEXT
    );

CREATE UNIQUE INDEX unique_route_index
    ON route (source, destination);

If the unique_route_index is corrupt, you can find duplicated rows in the route table using this query:

SELECT
    source,
    destination,
    count
FROM
    (SELECT
        source,
        destination,
        COUNT(*) AS count
    FROM route
    GROUP BY
        source,
        destination) AS foo
WHERE count > 1;

The query groups the data by the same source and destination fields defined in the index, and filters any entries with more than one occurrence.

Resolve the problematic entries in the rows by manually deleting or merging the entries until no duplicates exist. After all duplicate entries are removed, you can use the REINDEX command to rebuild the index.