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There must be ' <' and '> ' ordering operators having the same left and right input datatypes as the mergejoinable operator itself . What this means is that a join using " = " between interval fields would yield different results if implemented as a hash join than if implemented another way , because a large fraction of the pairs that should match will hash to different values and will never be compared by the hash join . Again , the reasoning is that if a notify were delivered within a transaction that was later aborted , one would want the notification to be undone somehow - - - but the backend cannot " take back " a notify once it has sent it to the frontend . The check for accessing phone_number is still performed , so nobody than the secretary can use it . However , unless you have a special reason for doing this ( such as using OIDs as keys in tables ) , do n't do it . Unless you have problems later on , these messages may be safely ignored . Disk ManagementAlternate Locations It is possible to create a database in a location other than the default location for the installation . Firstly , if a NOTIFY is executed inside a transaction , the notify events are not delivered until and unless the transaction is committed . These operators must be named ' <' and '> ' ; you do not have any choice in the matter , since there is no provision for specifying them explicitly . But once it starts down the stairs and gravity affects it , the potential energy is converted to the energy of motion , or kinetic energy , and the Slinky gracefully tumbles coil by coil down the stairs .