ZomboDB is capable of answering fulltext queries even when Postgres decides to plan the query using a sequential scan.
An example of such a query and its plan is:
tutorial=# SELECT * FROM products
WHERE zdb('products', products.ctid) ==> 'round';
id | name | keywords | short_summary | long_description | price | inventory_count | discontinued | availability_date
----+----------------+-------------------------+--------------------------------+----------------------------------------------------------------------------------------------+-------+-----------------+--------------+-------------------
1 | Magical Widget | {magical,widget,round} | A widget that is quite magical | Magical Widgets come from the land of Magicville and are capable of things you can't imagine | 9900 | 42 | f | 2015-08-31
2 | Baseball | {baseball,sports,round} | It's a baseball | Throw it at a person with a big wooden stick and hope they don't hit it | 1249 | 2 | f | 2015-08-21
(2 rows)
tutorial=# explain SELECT * FROM products
WHERE zdb('products', products.ctid) ==> 'round';
QUERY PLAN
---------------------------------------------------------------
Seq Scan on products (cost=0.00..1.15 rows=2 width=153)
Filter: (zdb('products'::regclass, ctid) ==> 'round'::text)
(2 rows)
A "Seq Scan" is a Postgres query plan construct where it evaluates the query against each row in the table, one-at-a-time.
While Postgres is determining which rows to return, ZomboDB detects the first time it sees the zdb('products', products.ctid) ==> 'round' query condition and queries Elasticsearch (using an endpoint exposed by ZomboDB's ES plugin). ZomboDB then builds an in-memory hashtable (within the Postgres session backend) of all matching rows.
Once the hashtable is built, the current ctid of the sequential scan is looked up in the hashtable. If it's found, that row matches. If not, then the row doesn't match.
If you consider a sequential scan over, say, a 1M row table, there will be one query to Elasticsearch to find the matching rows, and then 1M individual hashtable lookups.