Should I create unique index if a column contains unique constraint and I want to fast search by this column?
For example I have a table users
with column login
that should be unique. I need fast search user by the login
column.
Which is the best way to do it:
WHERE login = 'something'
?)Second case is unique login
on not locked users (column locked = false
). Postgres does not support partial conditions. Should I create a unique conditional and a partial index or is only a partial index enough?
And one more question: should I create new index for a column with a foreign key? For example: users.employee_id
relates to employees.id
, should I create an index on employee
column for optimized query SELECT * FROM users WHERE employee_id = ....
? When are internal indexes used by the optimization engine and when not?
A constraint has different meaning to an index. It gives the optimiser more information and allows you to have foreign keys on the column, whereas a unique index doesn't.
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. 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.
Yes, absolutely. A unique constraint creates a unique index.
Index - improves the performance of retrieval and sort operations on Table data. Unique Constraints - a combination of values that uniquely identify a row in the Table.
I have a table 'users' with column login that should be unique
If this is the case you need a unique constraint. Unique constraints are enforced (behind the scenes) by unique indexes.
Conceptually:
Is it used in select queries with WHERE login = 'something'?
Yes, it is.
Second case is unique login on not locked users (column locked = false).
If this is the case a unique
constraint won't work. Maybe a trigger on insert could help here.
should I create new index for column with foreign key?
No, it's not needed (at least in the current version 10
and perhaps the later versions), s. documentation:
PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. [...] There's no need to manually create indexes on unique columns; doing so would just duplicate the automatically-created index.
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