How Can I Efficiently Remove Duplicate Rows from a Large Table with a Unique Constraint in PostgreSQL?

Efficiently Removing Duplicates from a Large Table with a Unique Constraint in PostgreSQL

Adding a unique constraint to a large table (millions of rows) containing duplicates presents a significant challenge due to the time-consuming nature of standard SQL DELETE statements. This article explores a more efficient solution.

A highly effective method leverages a PostgreSQL extension and the USING clause. Consider this query:

<code class="language-sql">DELETE FROM table USING table AS alias
WHERE table.field1 = alias.field1 AND table.field2 = alias.field2 AND table.max_field < alias.max_field;</code>

This query cleverly removes duplicate rows. It identifies duplicates based on field1 and field2, and retains the row with the highest max_field value, deleting those with lower values.

For instance, to enforce uniqueness on the email column in a user_accounts table, keeping the most recently created account (highest id), the query would be:

<code class="language-sql">DELETE FROM user_accounts USING user_accounts AS ua2
WHERE user_accounts.email = ua2.email AND user_accounts.id < ua2.id;</code>

The USING clause is key to the efficiency of this PostgreSQL-specific approach. It significantly improves performance compared to traditional DELETE methods when dealing with massive datasets.

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