![]() ![]() ![]() This obviously prevents us from adding the same value more than once:ĮRROR: duplicate key value violates unique constraint "i"īut what about NULLs? NULL means undefined, so what should the unique index do when you try to insert more than one row with the value of NULL? Are two NULL values the same or not? If you ask PostgreSQL if NULL equals NULL, you get undefined (or NULL):Ĭomparing something which is not defined against something which is not defined does not make much sense. Postgres=# create unique index i on t ( a ) ALTER TABLE mytable ADD CONSTRAINT uniquecolumnname UNIQUE (columnname) In above statement, mytable is the name of the table you want to modify, columnname is the name of the column you. What is it about? The answer is quite easy if we do a little example which shows the effect of this new feature.Ĭonsider the following table and unique index: Unique Indexes Indexes can also be used to enforce uniqueness of a columns value, or the uniqueness of the combined values of more than one column. To make a column unique in a PostgreSQL table, you can use the ALTER TABLE statement with the ADD CONSTRAINT clause. Now the questioner asked what the difference is. Postgres 13 (original answer) Store the hash value in a generated column and create a UNIQUE. So just: CREATE UNIQUE INDEX bankhashuni ON bank (hashrecordextended((col1, col2, col3, col4, col5, col6, col7, col8),0)) That's all. The type argument is available for configuring the index type in PostgreSQL with. Now, an expression index seems more attractive than a generated column. Another nice feature was just committed for PostgreSQL 15: Add UNIQUE null treatment option (discussion here, commit details here). A non unique index allows multiples of the same value in that column. Available on the unique, unique and index attributes in all databases.
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