Thread: Many-To-Many Bridge Table Index

Hi guys,

I'd really appreciate if you could clarify this to me. Let's say I have
a table named TABLE_A that is a bridge to many-to-many relationship
between TABLE_B and TABLE_C, as such here is my declaration:

CREATE TABLE table_a (
      table_b_id    INT4        NOT NULL
    REFERENCES table_b ON DELETE CASCADE ON UPDATE CASCADE,

    table_c_id    INT4        NOT NULL
    REFERENCES table_c ON DELETE RESTRICT ON UPDATE CASCADE,

    CONSTRAINT table_a_pkey
    PRIMARY KEY (table_b_id, table_c_id)
);

This will automatically create a unique index on the primary key.

Well, if I do a query later like:

select * from table_a where table_b_id=1,

will the query use an index? Will this be true when I do join on this table?

Thanks so much,

Serge

Am Donnerstag, 29. September 2005 21:44 schrieb Sergei Dubov:
> select * from table_a where table_b_id=1,
>
> will the query use an index? Will this be true when I do join on this
> table?

It could use the index, but whether it will depends on the particular data
distribution.  The EXPLAIN command is your friend.

--
Peter Eisentraut
http://developer.postgresql.org/~petere/

Sergei Dubov wrote:
>
> This will automatically create a unique index on the primary key.
>
> Well, if I do a query later like:
>
> select * from table_a where table_b_id=1,
>
> will the query use an index? Will this be true when I do join on this
> table?

Yes to both (if the query plan thinks that is the most efficient way).

Of course a query on "table_c_id" can't use the index.
--
   Richard Huxton
   Archonet Ltd