Re: [SQL] Need indexes on inherited tables?

[Franco Bruno Borghesi]

Table T is not inheriting any indexes, neither the primary key constraint. That means that the search is going to use an index scan on table B and a sequential scan on table T (unless of course you add a primary key constraint or an index on table T).

You can check this things doing:
->SET enable_seqscan TO off;
->EXPLAIN SELECT * FROM B WHERE id=5;
you'll see an index scan on table B and sequential scans on the other tables.

Doing:
->SELECT C.relname AS table_name, C2.relname AS index_name FROM pg_index I LEFT JOIN pg_class C ON (I.indrelid=C.oid) LEFT JOIN pg_class C2 ON (C2.oid=I.indexrelid) WHERE  C.relname ILIKE '<table_name>'

you can find out what indexes are available for table_name (or \d <table_name> in psql).


On Sat, 2004-06-26 at 16:29, Phil Endecott wrote:

Dear Postgresql experts,

I have a base table that declares a primary key spanning a couple of columns:

create table B (
  id integer,
  xx someothertype,
  .....
  primary key (id, xx)
);

and a number of derived tables that inherit from B:

create table T (
 ....
) inherits (B);

An index is automatically created for B because of the primary key.

If I search for something in T using the key columns, e.g. I do

  select * from T where id=1 and xx=something;

will the index be used?  Or must I explicity create an index on id and xx for T and each of the other derived tables?

Is it any different if I search in B and find rows that are actually in T?

(Slightly unrelated: does the index on (id,xx) help when I am searching only on id?)

Thanks for any insight anyone can offer.

--Phil.

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