Pushdown $eq on 'String' not working properly

[shenyi0828]

FerretDB version

v1.20.1

Backend

PostgreSQL

Environment

• OS: CentOS8
• Deployment: ghcr.io/ferretdb/ferretdb:1.20.1
• Deployment details: k8s 1.25.x

What did you do?

I ran db.my_collection.findOne({'some_key' : 'match_value'}), it took many time to complete . And I found output like below:

Query	{"commandTag": "SELECT 1", "pid": 44279, "sql": "SELECT  _jsonb FROM \"my_db\".\"my_collection_xxxx\" WHERE _jsonb->$1 @> $2", "args": ["some_key","\"some_value\""], "time": "457.106837ms"}

note that it use @> (means "contains") other than = operator.

I ran explain:
db.runCommand({ explain: { findOne: "my_collection", query: { "some_key": {$eq: "match_value"}} }} )

What did you expect to see?

"filterPushdown" : true

What did you see instead?

"filterPushdown" : false

[AlekSi]

That should be solved by the upcoming 2.0