merger existence read: UNION the id and lk-lookup probes instead of OR-ing them across the hierarchy join — the OR is un-indexable and full-scans the subtype table on every child merge (one scan per relationship-edge write; surfaced by Castleberry onboarding); each UNION arm uses its own index (pk / lk_) and dedup preserves TOO_MANY_LOOKUP_ROWS semantics

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/merger/mod.rs

@@ -744,9 +744,19 @@ impl Merger {
       return Ok(None);
     }
 
-    let where_clause = format!("WHERE {}", where_parts.join(" OR "));
-
-    let final_sql = format!("{} {}", fetch_sql_template, where_clause);
+    // An OR across the hierarchy join is un-indexable — it forces a full scan
+    // of the subtype table on every child merge. UNION lets each arm use its
+    // own index (pk for the id arm, the lk_ unique index for the lookup arm)
+    // and still dedups, so TOO_MANY_LOOKUP_ROWS semantics are preserved.
+    let final_sql = if where_parts.len() == 1 {
+      format!("{} WHERE {}", fetch_sql_template, where_parts[0])
+    } else {
+      where_parts
+        .iter()
+        .map(|p| format!("{} WHERE {}", fetch_sql_template, p))
+        .collect::<Vec<_>>()
+        .join(" UNION ")
+    };
 
     let fetched = match self.db.query(&final_sql, None) {
       Ok(Value::Array(table)) => {