Slow Execution of ALTER TABLE on Large Table when changing column type

Last edited: 9/9/2025

If you encounter slow execution of the ALTER TABLE operation on a large table when changing a column data type, consider the following alternative approach.

Alternative Approach:

  1. Add a New Column with the New Type: ALTER TABLE "table_name" ADD COLUMN "new_column_name" new_data_type;

  2. Copy Values from the First Column to the Second: UPDATE "table_name" SET "old_column_name" = "new_column_name"::new_data_type;

  3. Drop the Old Column: ALTER TABLE "table_name" DROP COLUMN "old_column_name";

Why Use This Approach? The long execution time for the ALTER TABLE operation can be attributed to the large size of the table. This segmented approach helps in:

  • Efficiency: The process is more efficient as it avoids prolonged transactions.
  • Minimizing Disruption: Migrating data systematically minimizes the impact on other operations and users.

Additional Recommendations:

  • Set session statement_timeout to 0 to prevent potential transaction timeouts.

  • Monitor currently blocked database transactions during the process using the provided script:

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create view  public.lock_monitor asselect  coalesce(    blockingl.relation::regclass::text,    blockingl.locktype  ) as locked_item,  now() - blockeda.query_start as waiting_duration,  [blockeda.pid](http://blockeda.pid/) as blocked_pid,  blockeda.query as blocked_query,  blockedl.mode as blocked_mode,  [blockinga.pid](http://blockinga.pid/) as blocking_pid,  blockinga.query as blocking_query,  blockingl.mode as blocking_modefrom  pg_locks blockedl  join pg_stat_activity blockeda on [blockedl.pid](http://blockedl.pid/) = [blockeda.pid](http://blockeda.pid/)  join pg_locks blockingl on (    blockingl.transactionid = blockedl.transactionid    or blockingl.relation = blockedl.relation    and blockingl.locktype = blockedl.locktype  )  and [blockedl.pid](http://blockedl.pid/) <> [blockingl.pid](http://blockingl.pid/)  join pg_stat_activity blockinga on [blockingl.pid](http://blockingl.pid/) = [blockinga.pid](http://blockinga.pid/)  and blockinga.datid = blockeda.datidwhere  not blockedl.granted  and blockinga.datname = current_database();

Notes: Execute these steps during a maintenance window or low-traffic period to minimize disruption.