Module describe 

v6.3.3 — Describe statement pre-Execute.

Given a Statement returned by Engine::prepare(), compute (parameter_oids, output_columns) without executing the statement.

Implementation policy:

• parameter_oids: count distinct $N placeholders in the AST and return a Vec of zeros (oid=0 = “let the server infer at Bind time”). PG drivers happily accept this.
• output_columns: resolve the FROM namespace — table, view, CTE, derived table, and every joined relation — then describe each SELECT item against it. Anything that cannot be resolved collapses the whole list to empty, which the pgwire layer maps to a NoData reply.

v7.39 (round 462) — “complex shapes degrade to NoData, which drivers tolerate” was wrong, and the cost was silent.

Execute never sends a RowDescription (Describe owns it), so a shape Describe cannot resolve reaches an extended-protocol client as data rows with NO column metadata at all. Measured against PG18 over sqlx: a view, a JOIN, a derived table, a UNION, a CTE and every system catalog view all declared zero columns, so row.get(0) was out of bounds on rows that plainly carried values. Only a bare single-table SELECT worked. PG18 declares all of them.

Describe and execution must therefore agree by construction — [crate::tests] pins the two against each other over a shape corpus so a future shape cannot drift the way views did.

Functions

describe_prepared

One-shot describe of a prepared Statement.