Different between single and multiple row indexing

With the following 3 indexes, the "AND" query will use the index with multiple rows, whilst the "OR" query will use the combination of single indexes.

CREATE INDEX user_email_id_index
  ON action_audits
  USING btree
  (user_email COLLATE pg_catalog."default", user_id);

CREATE INDEX user_email_index
  ON action_audits
  USING btree
  (user_email COLLATE pg_catalog."default");

CREATE INDEX user_index
  ON action_audits
  USING btree
  (user_id);