IIRC, this isn&#x27;t true for Postgres.  Many common schema alterations in Posgres are lock-free (non-blocking).  In MySQL, these alterations used to be locking (ie. O(n) in number rows)... but perhaps it has gotten better? (Wouldn&#x27;t know; don&#x27;t use MySQL.) I did a quick cursory google search to find some substantiation:<p>- <a href="http:&#x2F;&#x2F;www.estelnetcomputing.com&#x2F;index.php?&#x2F;archives&#x2F;12-Locking-when-altering-a-table-in-postgres-vs.-mysql.html" rel="nofollow">http:&#x2F;&#x2F;www.estelnetcomputing.com&#x2F;index.php?&#x2F;archives&#x2F;12-Lock...</a><p>- <a href="http:&#x2F;&#x2F;stackoverflow.com&#x2F;a&#x2F;6542757" rel="nofollow">http:&#x2F;&#x2F;stackoverflow.com&#x2F;a&#x2F;6542757</a>