Comment 1985 added to page dml-update.html of version 7.4

Author: puco <puco(at)atlas(dot)sk>
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It looks like that Postgresql is checking for integrity and consistency during the update and not only after the end of update.

For example a have a table with a unique index on row Rank, that has 3 rows with values (0,1,2). When I want to execute this statement:

update
RankTable
set
Rank=(case when Rank=2 then 0 else Rank+1 end)

The table is inconsistent in the middle of update but when the update ends the table is again consistent. I understand that this should be correct and also passes in Microsoft SQL Server. Is there a possibility to force PostgresSQL to execute this statement without an error saying that I'm violating a constraint with duplicate key.
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Manual page: http://wwwmaster.postgresql.org/docs/7.4/interactive/dml-update.html
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