Postgres unique constraint without index

The index covers the columns that make up the primary key or unique constraint (a multicolumn index , if appropriate), and is the mechanism that enforces the constraint. When an index is declared unique , multiple table rows with equal indexed values are not allowed. Null values are not considered equal.

A multicolumn unique index will only reject cases where all indexed columns are equal in multiple rows. Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. A uniqueness restriction covering only some rows cannot be written as a unique constraint , but it is possible to enforce such a restriction by creating a unique partial index.

Create unique constraint with null columns. This means that disabling all constraints on the table does not refer to unique constraints. In other words it is not possible to insert duplicate data into the table by disabling a unique constraint. Generally there is no functional difference between a unique index and a unique constraint.

Indexes can also be associated with a constraint , as long as: The index cannot have expression columns nor be a partial index. Which leads me to believe there is currently no way to have a unique index with. I know the thumb rule of UNIQUE Constraint is, UNIQUE Key column can be a NULL.

They are functionally the same and unique constraint will create a unique index.

The uniqueness property is a constraint and so a unique index without a corresponding constraint is an improper model. A primary key is a column or a group of columns used to identify a row uniquely in a table. You define primary keys through primary key constraints. But NULL is not a comparable value and unique constraint ignores these values. Sometime we would to allow only one NULL in column.

UNIQUE constraint ensures unique values in column(s). There is a simply solution based on partial index and functional (here only constant) index. Distinguishing between NULL values is impossible, as per SQL standard. Unique Constraints and Check Constraints.

These are my favorite workarounds for one and multiple columns. The short version is that NULL represents missing information and comparing a field with missing information with another makes no sense. Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. Therefore you should always create the constraint at the database level - either with an index or a unique constraint.

There is little distinction between unique indexes and unique constraints. ALTER TABLE LOCATION ADD CONSTRAINT dist. The Value of the column must be unique across the whole table.

However, the column can have many NULL values because PostgreSQL treats each NULL value to be unique. Regardless of using unique constraint or unique index , the field can accept null values, however the uniqueness will result in only accepting a single row with null value.

The solution to allow nulls in unique fields is create a unique filtered index excluding the nulls of the index , due to that the uniqueness of the nulls will not be validated. Note: The syntax of CREATE TABLE applies only to Postgres -XC. Postgres -XC automatically creates an index for each unique constraint and primary key constraint to enforce uniqueness. Thus, it is not necessary to create an index explicitly for primary key columns.

Modern day applications need to persist their state with a database. Today we will be taking a look at the importance of database constraints and how to test them using pgTAP. Under a period of high load it seems that creating a temporary tables with the same name in separate postgres back-ends can trigger this error: ERROR: duplicate key violates unique constraint pg_type_typname_nsp_ index The command that causes it is: create temp table register_reqs(full_key text, register_ index integer) WITHOUT OIDS ON COMMIT DROP Is this being caused by a race.

PG you could make use of the DO statement to run this. Adding an exclusion constraint builds the supporting index , an unfortunately, there is currently no support for using an existing index (as you can do with a unique constraint ).