Indexes on PostgreSQL hstore array columns

I know you can create an index on a field in a hstore column. I know you can also create a GIN index on a array column.

But what is the syntax to create an index on an hstore array?

e.g.

CREATE TABLE customer (
    pk serial PRIMARY KEY,
    customer hstore,
    customer_purchases hstore[]
);

Let's say the customer purchases hstore may be a hash like

productId -> 1
price -> 9.99

and I have an array of those in the customer_purchases hstore[]

I want to create an index on customer.customer_purchases[]-> productId

Is this possible? I've tried different combinations of CREATE INDEX syntaxes and none of them seem to support indexing fields in an hstore array.

Solution

I think you've misunderstood PostgreSQL Arrays. An Array is actually just a string. You can't index the objects (in this case HSTOREs) in the array, simply because it's not a TABLE.

Instead, create an extra table:

CREATE TABLE customer (
    pk bigserial PRIMARY KEY,
    customer hstore
);

CREATE TABLE purchases (
    pk bigserial PRIMARY KEY,
    customer_pk bigint not null,
    purchase hstore not null,
    constraint "must be a valid customer!" 
        foreign key (customer_pk) references customer(pk)
);

Also, Why are you using HSTOREs here?

If you must create an INDEX based on the "purchase" HSTORE here, do something like this:

CREATE OR REPLACE FUNCTION purchase_amount(purchase hstore) returns float as $$
    select ($1 -> 'price')::float;
$$ language 'SQL' IMMUTABLE;

CREATE INDEX "purchases by price" ON purchases (purchase_amount(purchase));

Is this just an exercise to understand the HSTORE type? or do you have some real use case that would make all this obfuscation of your real data worthwhile?