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If the referencing and referenced columns have different collations, the parser will be unable to resolve which collation to use unless it's helped out in this way. The effects are sometimes masked, if we end up using a non-collation-sensitive plan; but if we do use a mergejoin we'll see a failure, as recently noted by Robert Haas. The SQL spec states that the referenced column's collation should be used to resolve RI checks, so that's what we do. Note however that we currently don't append a COLLATE clause when writing a query that examines only the referencing column. If we ever support collations that have varying notions of equality, that will have to be changed. For the moment, though, it's preferable to leave it off so that we can use a normal index on the referencing column. |
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PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: http://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Changes between all PostgreSQL releases are recorded in the file HISTORY. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.