Query: the missing key refusal comes before the filter syntax and unused value checks when the key condition has an equality #712

Measured on Amazon DynamoDB on 2026-10-31 between 23:04 and 23:06 UTC in us-east-1, each request twice with the same answer. The ExtendDB answers are from a SQLite server built from main at f49c5f823.

When a key condition misses an equality on a key attribute, the service reports an invalid FilterExpression and an unused value ahead of the key schema refusal. ExtendDB reports the key schema refusal first when the key condition has an equality clause on another attribute. #479 makes ExtendDB report the filter and unused value faults first for a key condition with no equality clause; the rows below have one.

Table with HASH pk and RANGE sk (both S), and a GSI with HASH g1 and g2 (both S) and no RANGE. :x and :y are S values.

handle_query (crates/engine/src/query.rs) classifies the key condition with resolve_key_schema_with and runs the single-attribute missing key check before it parses the FilterExpression and before the unused value check. The key value type check runs in the same block, and #544 moves it after the filter, projection, unused value and Select checks; the missing key and HASH equality refusals stay where they are.

The same position shows with #558's filter operator count: on the table above, pk = :p AND sk = :n with :n an N value and a FilterExpression of 301 operators answers Invalid FilterExpression: The expression contains too many operators; operator count: 301 on the service (measured 2026-10-31 23:10 UTC, twice) and the type sentence with #558, because the type check runs first.