Set up Alter Index Unusable in PostgreSQL - What steps are necessary ?

PostgreSQL

With IbmiMedia's Postgresql support services, let's gain more insight into the alter index unusable Postgresql feature.

What does the alter index unusable command do in PostgreSQL ?

Unlike some other database systems, PostgreSQL does not support the "ALTER INDEX...UNUSABLE" command. To get a similar outcome, we can either remove and rebuild the index or use partial indexes.

How does this work ?

1. Dropping the existing index and recreating a new index

We can drop an index in PostgreSQL and rebuild it later to make it temporarily ineffectual. Dropping an index takes it away from the database, while rebuilding it puts back the index with the stated columns and parameters.

In PostgreSQL, to discard and rebuild an index, execute the following SQL:

-- Drop the index
DROP INDEX index_name;
-- Recreate the index
CREATE INDEX index_name ON table_name (column_name);

Although it is possible to make the index useless by erasing it, one should be mindful of its effect on query performance before doing so. Therefore, caution must be taken when deleting an index.

2. Utilizing partial indexes

Creating partial indexes is a different means of achieving similar results. Partial indexes are ones that solely include a subset of rows that fulfill a specified requirement.

Additionally, we can build a "unusable" index by implementing a condition that will always be false.

To create a partial index in PostgreSQL, write:

sql
--  Create an index named 'index_name' on 'table_name' (column_name) with a condition that always evaluates to false by writing: 
SQL 
CREATE INDEX index_name ON table_name (column_name) WHERE 1 = 0;

The index in this example is rendered practically “unusable” as it offers no value for query optimization due to the criterion 1 = 0 always evaluating to false, which means it does not include any rows.

When attempting to replicate the outcome of labeling an index as useless in PostgreSQL, it is imperative to weigh the implications of performance and query optimization that come with removing and rebuilding the index or utilizing partial indexes with criteria that always evaluate to false.

Some reasons you may want to temporarily disable (mark unusable) an index include:

Performing bulk loads: Disabling indexes during bulk loads can improve loading performance.
Database maintenance: Disabling indexes can improve the performance of certain database maintenance operations like vacuum, cluster, reindex, etc.
Testing: Disabling an index allows you to test how queries perform without that index. This can help determine how useful the index really is.