Module placeholder_types 

Best-effort type inference for $N prepared-statement placeholders.

The pgwire extended-query protocol lets a client send Parse without declaring any parameter OIDs. The server then has to answer Describe with a ParameterDescription, and answering “unknown” (OID 0) for every position forces well-behaved clients into a failure: tokio-postgres refuses to serialize an i64 against an unknown OID. This module walks the unsubstituted statement and reports the positions whose type the SQL — plus the catalog the SQL names — pins down.

Why a separate walk rather than planner output

The planner cannot plan an AST that still contains placeholders — the resolver has no Value::Placeholder arm — and the Control Plane’s schema-inference pass therefore rewrites $N to NULL in the SQL text before planning. That rewrite destroys the position → type link, so inference has to happen here, on the parsed-but-unbound AST.

Directionality: under-infer, never over-infer

A PostgreSQL client that receives an unknown parameter type sends the value in text format, which the bind layer already handles — so leaving a position unresolved only costs a text round-trip. Reporting a concrete OID, by contrast, makes the client commit to that type’s binary encoding. Any position whose type is not pinned down unambiguously stays None.

Structs

InferredParamType

A parameter type the statement pins down.

Functions

infer_placeholder_types

Best-effort, catalog-aware inference of $N placeholder types.