Function json_object_expr 

fn json_object_expr(columns: &[String]) -> Expr

Builds a JSONB object from columns by concatenating a JSON text and parsing it — GreptimeDB has no struct→json function. Keys are JSON-escaped in Rust; values are JSON-escaped at runtime via json_escaped_value_expr.

TODO(entity-graph): replace the text round-trip with a UDF that assembles JSONB directly from the value columns (jsonb::ObjectBuilder, keys baked in), dropping the escaping helpers and the per-row parse cost.