Обсуждение: Check for existence of index

I have a script that automatically creates my database objects.  In
order to automatically create indexes, it needs to first make sure they
don't exist.

For things like tables, this is easy:

select * from information_schema.tables where table_schema =
"<myschema>" and table_name = "<tablename>"

But for indexes it is hard for some reason.  There's a catalog table
"pg_index", but it doesn't have index, schema or table names.  I
eventually found them in pg_class but the table and schema names aren't
there.

After some searching around, I came across this very strange (to me,
anyway) "::regclass" thing that let me do this:

select * from pg_catalog.pg_index where indexrelid =
'schema.index'::regclass

I'm not really clear what's that doing, but in any case it still isn't
what I want.  That query returns information when the index exists but
errors out when the index doesn't exist.  Is there a way I can get a
non-erroring query on either condition that will tell me if an index
exists on a given table in a given schema?

And another thing, can't I do this:

create table s.a (blah);
create table s.b (blah);

create index myindex on s.a(blah);
create index myindex on s.b(blah);

?  When I drop them I have to specify the schema name, so presumably it
tracks them that way.  Why can't I have the same index name be on
different tables?

David Rysdam wrote:

> I have a script that automatically creates my database objects.  In
> order to automatically create indexes, it needs to first make sure
> they don't exist.
>
> For things like tables, this is easy:
>
> select * from information_schema.tables where table_schema =
> "<myschema>" and table_name = "<tablename>"
>
> But for indexes it is hard for some reason.  There's a catalog table
> "pg_index", but it doesn't have index, schema or table names.  I
> eventually found them in pg_class but the table and schema names
> aren't there.
>
> After some searching around, I came across this very strange (to me,
> anyway) "::regclass" thing that let me do this:
>
> select * from pg_catalog.pg_index where indexrelid =
> 'schema.index'::regclass
>
> I'm not really clear what's that doing, but in any case it still isn't
> what I want.  That query returns information when the index exists but
> errors out when the index doesn't exist.  Is there a way I can get a
> non-erroring query on either condition that will tell me if an index
> exists on a given table in a given schema?
>
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David Rysdam <drysdam@ll.mit.edu> writes:
> Why can't I have the same index name be on
> different tables?

You can ... if they are in different schemas.  Indexes and tables share
the same namespace, ie, they must be unique within a schema.

As for your original question, you probably want something like

SELECT ... FROM pg_class c, pg_namespace n WHERE c.relnamespace = n.oid
  AND relname = 'indexname' AND nspname = 'schemaname' AND relkind = 'i';

If you actually want to verify that this index is on a specific table,
you'll need a more complicated join involving pg_index and a second
scan of pg_class.  See
http://www.postgresql.org/docs/8.0/static/catalogs.html

            regards, tom lane

Tom Lane wrote:

>David Rysdam <drysdam@ll.mit.edu> writes:
>
>
>>Why can't I have the same index name be on
>>different tables?
>>
>>
>
>You can ... if they are in different schemas.  Indexes and tables share
>the same namespace, ie, they must be unique within a schema.
>
>As for your original question, you probably want something like
>
>SELECT ... FROM pg_class c, pg_namespace n WHERE c.relnamespace = n.oid
>  AND relname = 'indexname' AND nspname = 'schemaname' AND relkind = 'i';
>
>If you actually want to verify that this index is on a specific table,
>you'll need a more complicated join involving pg_index and a second
>scan of pg_class.  See
>http://www.postgresql.org/docs/8.0/static/catalogs.html
>
>            regards, tom lane
>
>
Well, since I can't have more than one index of a given name in a schema
anyway, I'll have to name them "$tablename_$indexname" or something,
which means I won't have to verify they are on a particular table.

Anyway, this query looks good.  I was getting lost in all the
terminology ("namespace" vs "schema") data distributed all over (some
stuff in pg_index, some in pg_class, etc).