Create An Index Without Locking The Table

When creating an index for a column, the process of building the index will lock the column's table. For small datasets this isn't a concern because the index will take no time at all to create. For larger datasets, the lock could last long enough to create meaningful downtime. This can all be avoided by telling Postgres to build the index concurrently.

create index concurrently idx_book_isbns on books(isbn);

Creating the index this way will take a bit longer and put more strain on machine resources, but it allows concurrent inserts, updates, or deletes on the table. In other words, you can add an index to a large table in a production environment without bringing down your app.

Read more about the details and potential caveats in the docs.