indexing primary and foreign keys w/lookup table

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Hi All.

I was wondering...I currently have indexes on the primary key id and
foreign key id's for tables that resemble the following. Is this a
good idea/when would it benefit me? I don't want waste a lot of
unnecessary space on indexes.

CREATE TABLE stuff (
    id    BIGSERIAL PRIMARY KEY,
    stuff    TEXT
);
CREATE INDEX stuff_id ON stuff(id);

CREATE TABLE accounts (
    id        BIGSERIAL PRIMARY KEY,
    name        TEXT,
    email        TEXT,
);
CREATE INDEX accounts_id ON accounts(id);

CREATE TABLE stuff_by_account (
    account_id    BIGINT REFERENCES accounts(id),
    stuff_id    BIGINT REFERENCES stuff(id)
);
CREATE INDEX stuff_by_account_account_id ON stuff_by_account
(account_id);
CREATE INDEX stuff_by_account_stuff_id ON stuff_by_account(stuff_id);

do I need any/all of these indexes for my lookup table to work well?
I am thinking I can get rid of stuff_id and accounts_id. Thoughts?

- -Neal
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