SELECT COUNT(*) FROM t WHERE <predicate> — and any query that plans an
empty-projection scan — panics the executing query task:
InvalidArgumentError("must either specify a row count or at least one
column")
Root cause
MetadataEraserExec wraps every LanceDB table scan to strip schema-level
metadata, rebuilding each batch in execute():
RecordBatch::try_new(schema.clone(), batch.columns().to_vec()).unwrap()
RecordBatch::try_new infers the row count from the columns. COUNT(*)
with a filter is planned with an empty projection, so the scan emits
zero-column batches — there are no columns to infer a length from,
try_new returns Err, and the .unwrap() panics.
(This is specific to the empty-projection case: COUNT(*) with no filter
is answered from statistics and never scans, and COUNT(<col>) projects a
column — both already work.)
