When administering a PostgreSQL installation, we will eventually need to create new tables and indexes. In the case of new indexes, the table is locked in shared exclusive access mode for the duration of the creation process, blocking any insert, update, or delete activity. This both prevents inconsistencies, and allows the database to modify the table structure to reflect the new index.
Unfortunately, this process is fundamentally incompatible with maintaining a highly-available server. While building the index, PostgreSQL needs to examine every valid table row, which means loading it from the disk into memory. For large or active tables, this can cause excessive strain on the system. Other database activities will reduce available disk bandwidth, and the required lock will block all modifications of data in that table. Combined, this can lead to a table being locked for a very long time.
Beginning with PostgreSQL 8.2, indexes can be created concurrently...