refactor(postgres): simplify query plan handling

sqlx-postgres/src/connection/describe.rs

@@ -451,11 +451,15 @@ WHERE rngtypid = $1
 
         let mut nullables = Vec::new();
 
-        if let Explain::QueryPlan(query_plan @ QueryPlan { plan, .. }) = &explain {
-            if let Some(outputs) = &query_plan.plan.output {
-                nullables.resize(outputs.len(), None);
-                visit_plan(&plan, outputs, &mut nullables);
-            }
+        if let Some(
+            plan @ Plan {
+                output: Some(ref outputs),
+                ..
+            },
+        ) = &explain.plan
+        {
+            nullables.resize(outputs.len(), None);
+            visit_plan(plan, outputs, &mut nullables);
         }
 
         Ok(nullables)
@@ -488,19 +492,16 @@ fn visit_plan(plan: &Plan, outputs: &[String], nullables: &mut Vec<Option<bool>>
 }
 
 #[derive(serde::Deserialize)]
-#[serde(untagged)]
-enum Explain {
-    /// {"Plan": ...} -- returned for most statements
-    QueryPlan(QueryPlan),
-    /// The string "Utility Statement" -- returned for
-    /// a CALL statement
-    UtilityStatement(String),
-}
-
-#[derive(serde::Deserialize)]
-struct QueryPlan {
-    #[serde(rename = "Plan")]
-    plan: Plan,
+struct Explain {
+    // NOTE: the returned JSON may not contain a `plan` field, for example, with `CALL` statements:
+    // https://github.com/launchbadge/sqlx/issues/1449
+    //
+    // In this case, we should just fall back to assuming all is nullable.
+    #[serde(default)]
+    plan: Option<Plan>,
+    // It may also contain additional fields we don't care about, which should not break parsing:
+    // https://github.com/launchbadge/sqlx/issues/2587
+    // https://github.com/launchbadge/sqlx/issues/2622
 }
 
 #[derive(serde::Deserialize)]