From 74e04634aa8811d27cc8560fc794a21bb5b6a5d5 Mon Sep 17 00:00:00 2001
From: Jussi Saurio <jussi.saurio@gmail.com>
Date: Fri, 10 Oct 2025 13:12:53 +0300
Subject: [PATCH] Fix incorrectly using an equality constraint twice for index
 seek

Prevents something like `WHERE x = 5 AND x = 5` from becoming a two
component index key.

Closes #3656
---
 core/translate/optimizer/constraints.rs | 28 ++++++++++++++++++-------
 testing/join.test                       |  7 +++++++
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/core/translate/optimizer/constraints.rs b/core/translate/optimizer/constraints.rs
index 7e32f17c5..486c36687 100644
--- a/core/translate/optimizer/constraints.rs
+++ b/core/translate/optimizer/constraints.rs
@@ -486,7 +486,7 @@ pub fn usable_constraints_for_join_order<'a>(
             .map(|j| j.original_idx),
     );
     let mut usable: Vec<RangeConstraintRef> = Vec::new();
-    let mut last_column_pos = 0;
+    let mut current_required_column_pos = 0;
     for cref in refs.iter() {
         let constraint = &constraints[cref.constraint_vec_pos];
         let other_side_refers_to_self = constraint.lhs_mask.contains_table(table_idx);
@@ -498,6 +498,7 @@ pub fn usable_constraints_for_join_order<'a>(
             break;
         }
         if Some(cref.index_col_pos) == usable.last().map(|x| x.index_col_pos) {
+            // Two constraints on the same index column can be combined into a single range constraint.
             assert_eq!(cref.sort_order, usable.last().unwrap().sort_order);
             assert_eq!(cref.index_col_pos, usable.last().unwrap().index_col_pos);
             assert_eq!(
@@ -520,15 +521,28 @@ pub fn usable_constraints_for_join_order<'a>(
             }
             continue;
         }
-        if cref.index_col_pos != last_column_pos {
+        if cref.index_col_pos != current_required_column_pos {
+            // Index columns must be consumed contiguously in the order they appear in the index.
             break;
         }
         if usable.last().is_some_and(|x| x.eq.is_none()) {
+            // Usable index key must have 0-n equalities and then a maximum of 1 range constraint with one or both bounds set.
+            // If we already have a range constraint before this one, we must not add anything to it
             break;
         }
-        let constraint_group = match constraints[cref.constraint_vec_pos].operator {
+        let operator = constraints[cref.constraint_vec_pos].operator;
+        let table_col_pos = constraints[cref.constraint_vec_pos].table_col_pos;
+        if operator == ast::Operator::Equals
+            && usable
+                .last()
+                .is_some_and(|x| x.table_col_pos == table_col_pos)
+        {
+            // If we already have an equality constraint for this column, we can't use it again
+            continue;
+        }
+        let constraint_group = match operator {
             ast::Operator::Equals => RangeConstraintRef {
-                table_col_pos: constraints[cref.constraint_vec_pos].table_col_pos,
+                table_col_pos,
                 index_col_pos: cref.index_col_pos,
                 sort_order: cref.sort_order,
                 eq: Some(cref.constraint_vec_pos),
@@ -536,7 +550,7 @@ pub fn usable_constraints_for_join_order<'a>(
                 upper_bound: None,
             },
             ast::Operator::Greater | ast::Operator::GreaterEquals => RangeConstraintRef {
-                table_col_pos: constraints[cref.constraint_vec_pos].table_col_pos,
+                table_col_pos,
                 index_col_pos: cref.index_col_pos,
                 sort_order: cref.sort_order,
                 eq: None,
@@ -544,7 +558,7 @@ pub fn usable_constraints_for_join_order<'a>(
                 upper_bound: None,
             },
             ast::Operator::Less | ast::Operator::LessEquals => RangeConstraintRef {
-                table_col_pos: constraints[cref.constraint_vec_pos].table_col_pos,
+                table_col_pos,
                 index_col_pos: cref.index_col_pos,
                 sort_order: cref.sort_order,
                 eq: None,
@@ -554,7 +568,7 @@ pub fn usable_constraints_for_join_order<'a>(
             _ => continue,
         };
         usable.push(constraint_group);
-        last_column_pos += 1;
+        current_required_column_pos += 1;
     }
     usable
 }
diff --git a/testing/join.test b/testing/join.test
index c14b96ab4..6c6aa7314 100755
--- a/testing/join.test
+++ b/testing/join.test
@@ -384,3 +384,10 @@ do_execsql_test_on_specific_db {:memory:} left-join-using-null {
     select a, b from t left join s using (a, b);
 } {1|
 2|}
+
+# Regression test for: https://github.com/tursodatabase/turso/issues/3656
+do_execsql_test_on_specific_db {:memory:} redundant-join-condition {
+    create table t(x);
+    insert into t values ('lol');
+    select t1.x from t t1 join t t2 on t1.x=t2.x where t1.x=t2.x;
+} {lol}
\ No newline at end of file