Ent API: exists() by Expression

This is another privacy-unaware API call, similar to count().

Ent.exists(vc, { field: "...", ... }): boolean

Returns true if there isat least one Ent in the database matching the where condition. Works across multiple microshards too.

In terms of the logic, exists() call is similar to count() > 0 check, with two performance optimizations:

1. It uses EXISTS SQL clause, which doesn’t read more tuples from the database than needed (as opposed to count() aggregate).

2. During the run, it severely reduces the weight (basically, the probability) of seqscan to happen (with SET enable_seqscan=off directive merged with the query). I.e. it implies that you must have a good index covering the where condition.

As all API calls in Ent Framework, multiple parallel exists() calls are batched into a single SQL query:

const [exists1, exists2] = await Promise.all([
  EntTopic.exists(vc, { creator_id: "123" }),
  EntTopic.exists(vc, { updated_at: { $gt: new Date("2024-01-01") } }),
]);

This sends the following SQL query to the underlying database:

SET enable_seqscan=off;
SELECT EXISTS (SELECT true FROM topics WHERE creator_id='123')
  UNION ALL
SELECT EXISTS (SELECT true FROM topics WHERE created_at>'...')

The exists() call is even more useful to build custom privacy checks than count(), because it’s faster and almost guarantees using an index.