Some databases will create a unique index for you when you create a primary key.
Oracle will create one, but only if you haven’t already done that.
Postgres will ALWAYS create a unique index based on the primary key – so you should never do that as well, or the db will have to maintain two identical indexes.
(When we migrated our db design from Oracle we ended up with lots of duplicate indexes).
SQL> Select * from pg_indexes order by schemaname, tablename;
Phil Horder
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From: Thomas Poty [mailto:thomas.poty@gmail.com]
Sent: 23 March 2018 07:56
To: pgsql-general@lists.postgresql.org
Subject: primary key and unique index
I am migrating fromMySQL to Postgresql 9.6.
In MySQL a "show create table" gives me :
...
PRIMARY KEY (`ID`,`CountryCode`,`LanguageCode`),
UNIQUE KEY `unique_my table_4` (`ID`,`CountryCode`,`LanguageCode`),
...
So, In PostgreSQL, does it make sense to create a primary key AND a unique index based on the same columns?
Is PostgreSQL smart enough to use the unique index created for the primary key.
I know PostgreSQL can be based on a unique index to create a primary key but I also know it is possible to create several indexes on the same columns with the same order.
Thanks
Thomas
