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Thread: Many-To-Many Bridge Table Index

Many-To-Many Bridge Table Index

From
Sergei Dubov
Date:
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

Re: Many-To-Many Bridge Table Index

From
Dawid Kuroczko
Date:
On 9/30/05, Sergei Dubov <sdubov@gmail.com> wrote:
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.

Yes it will.  Primary keys in PostgreSQL are implemented
using unique indexes, one may say.
 
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?

The short answer is: yes.

The longer answer is: yes, when planner sees that the index is the most
efficient way to approach given query.  In some cases using sequentional
scan may be more efficient.

Try EXPLAIN ANALYZE SELECT ...; -- it will tell you what did the planer
choose.

   Regards,
       Dawid