Add Create An Index Without Locking The Table as a postgres til

postgres/create-an-index-without-locking-the-table.md

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+# 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.
+
+```sql
+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](https://www.postgresql.org/docs/current/sql-createindex.html#SQL-CREATEINDEX-CONCURRENTLY)
+in the docs.