From 518016c8440ddb347f791cb5fd151afabcb0c76e Mon Sep 17 00:00:00 2001
From: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Date: Wed, 29 Jul 2026 09:02:34 +0900
Subject: [PATCH] Ignore Used for constraints SQLite marked not usable

---
 sqlite3_opt_vtable.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/sqlite3_opt_vtable.go b/sqlite3_opt_vtable.go
index 9e916ea..695f38d 100644
--- a/sqlite3_opt_vtable.go
+++ b/sqlite3_opt_vtable.go
@@ -460,7 +460,11 @@ func goVBestIndex(pVTab unsafe.Pointer, icp unsafe.Pointer) *C.char {
 	slice := unsafe.Slice(info.aConstraintUsage, int(info.nConstraint))
 	index := 1
 	for i := range slice {
-		if res.Used[i] {
+		// SQLite returns "xBestIndex malfunction" when an argvIndex is
+		// assigned to a constraint it marked as not usable, so ignore
+		// Used for those; they may become usable on a later xBestIndex
+		// invocation for a different plan.
+		if res.Used[i] && csts[i].Usable {
 			slice[i].argvIndex = C.int(index)
 			slice[i].omit = C.uchar(1)
 			index++