Fix a join in the postgres_indexes view

What does this MR do and why?

Fix a join in the postgres_indexes view that didn't join index information properly when multiple indexes with the same name existed in different schemas. This could result in queries returning multiple rows for a single index, with potentially different index definitions.

How to set up and validate locally

1. Before running the migration from this MR, setup the database:

   create table main_schema_table (id serial not null primary key);
   create table gitlab_partitions_dynamic.gpd_schema_table (id serial not null primary key);
   create index testi1 on main_schema_table (id);
   create index testi1 on gitlab_partitions_dynamic.gpd_schema_table (id);

2. Verify that we see extra entries for each index:

   select schema, tablename, name, definition from postgres_indexes where name = 'testi1';
             schema           |     tablename     |  name  |                                     definition
   ---------------------------+-------------------+--------+------------------------------------------------------------------------------------
    public                    | main_schema_table | testi1 | CREATE INDEX testi1 ON public.main_schema_table USING btree (id)
    public                    | gpd_schema_table  | testi1 | CREATE INDEX testi1 ON gitlab_partitions_dynamic.gpd_schema_table USING btree (id)
    gitlab_partitions_dynamic | main_schema_table | testi1 | CREATE INDEX testi1 ON public.main_schema_table USING btree (id)
    gitlab_partitions_dynamic | gpd_schema_table  | testi1 | CREATE INDEX testi1 ON gitlab_partitions_dynamic.gpd_schema_table USING btree (id)
   (4 rows)

3. Run the migration

   rails db:migrate:up VERSION=20211018161447

4. Verify that we only see a single entry per index per schema:

   select schema, tablename, name, definition from postgres_indexes where name = 'testi1';
             schema           |     tablename     |  name  |                                     definition
   ---------------------------+-------------------+--------+------------------------------------------------------------------------------------
    public                    | main_schema_table | testi1 | CREATE INDEX testi1 ON public.main_schema_table USING btree (id)
    gitlab_partitions_dynamic | gpd_schema_table  | testi1 | CREATE INDEX testi1 ON gitlab_partitions_dynamic.gpd_schema_table USING btree (id)
   (2 rows)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.