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I would create a stored procedure for the update in do inside of it:<br />
<br />
update all is_master to null<br />
insert new row with is_master ='t'<br />
<br />
surround this with a transaction + COMMIT / ROLLBACK and you done....
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I would create a stored procedure for the update in do inside of it:

update all is_master to null
insert new row with is_master ='t'

surround this with a transaction + COMMIT / ROLLBACK and you done....

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Postgres 9.0<br />
<br />
I want to have a trigger for an update on TableA, in that trigger, part of the process is to update other rows in TableA.<br />
<br />
What is the best way/method to do this without causing a loop.<br />
<br />
Simple example, Table A has field &quot;is_master&quot; which I want only 1 record of x,y,z &nbsp;to have is_master = 't'. When a new row is inserted and is x,y,z (and also newer version), I want for new row to have is_master = 't', and the old row is_master = null. <br />
<br />
The table already has a unique constraint on (x,y,z,is_master).<br />
<br />
Somehow, I need to set old is_master to null so I can insert new row with is_master = 't';
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Postgres 9.0

I want to have a trigger for an update on TableA, in that trigger, part of the process is to update other rows in TableA.

What is the best way/method to do this without causing a loop.

Simple example, Table A has field "is_master" which I want only 1 record of x,y,z  to have is_master = 't'. When a new row is inserted and is x,y,z (and also newer version), I want for new row to have is_master = 't', and the old row is_master = null. 

The table already has a unique constraint on (x,y,z,is_master).

Somehow, I need to set old is_master to null so I can insert new row with is_master = 't';

Postgres Update Triggers (an update in an update)

PostgreSQL is an object-relational database management system with an emphasis on extensibility and standards-compliance. As a database server, its primary function is to store data securely, supporting best practices, and to allow for retrieval at the request of other software applications. PostgreSQL implements the majority of the SQL2011 standard, is ACID-compliant and transactional using multiversion concurrency control (MVCC), and handles complex SQL queries using many indexing methods that are not available in other databases. It is cross-platform and runs on many operating systems including Linux, FreeBSD, OS X, Solaris, and Microsoft Windows.