From aae2ff034b5eb7e4091d0fb2db3300d21696a51c Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Fri, 11 Sep 2026 18:43:45 +0200
Subject: [PATCH] test(sql): cover
 explainOverACompoundQueryDescribesTheSetOperatorAndB...

---
 .../store/sql/DiskExecutorExplainTest.kt      | 82 +++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/src/test/kotlin/kestrel/store/sql/DiskExecutorExplainTest.kt b/src/test/kotlin/kestrel/store/sql/DiskExecutorExplainTest.kt
index e270dcc..3cd048c 100644
--- a/src/test/kotlin/kestrel/store/sql/DiskExecutorExplainTest.kt
+++ b/src/test/kotlin/kestrel/store/sql/DiskExecutorExplainTest.kt
@@ -314,3 +314,85 @@ class DiskExecutorExplainTest {
         assertEquals("  Offset 1", plan[1])
     }
 
+    @Test
+    fun explainOverACompoundQueryDescribesTheSetOperatorAndBothSides() {
+        val db = newDatabase()
+        db.run("CREATE TABLE t1 (id INTEGER PRIMARY KEY, name TEXT NOT NULL)")
+        db.run("CREATE TABLE t2 (id INTEGER PRIMARY KEY, name TEXT NOT NULL)")
+        db.run("INSERT INTO t1 VALUES (1, 'ada')")
+        db.run("INSERT INTO t2 VALUES (2, 'linus')")
+        val plan = db.explain("EXPLAIN SELECT name FROM t1 UNION SELECT name FROM t2")
+        assertEquals("Union", plan.first())
+        assertTrue(plan.any { it.trim() == "-> Seq Scan on t1" })
+        assertTrue(plan.any { it.trim() == "-> Seq Scan on t2" })
+    }
+
+    @Test
+    fun explainOverAWithRecursiveQueryDescribesTheCteAndItsBody() {
+        val db = newDatabase()
+        db.run("CREATE TABLE dual (x INTEGER)")
+        db.run("INSERT INTO dual VALUES (1)")
+        val plan = db.explain(
+            "EXPLAIN WITH RECURSIVE counter(n) AS (" +
+                "SELECT 1 FROM dual UNION ALL SELECT n + 1 FROM counter WHERE n < 5" +
+                ") SELECT n FROM counter"
+        )
+        assertTrue(plan.any { it == "CTE counter" })
+        assertTrue(plan.any { it == "Seq Scan on counter" })
+    }
+
+    @Test
+    fun explainOverAViewDescribesTheOuterQueryAgainstTheViewsMaterializedSchema() {
+        val db = newDatabase()
+        db.seedUsers()
+        db.run("CREATE VIEW adults AS SELECT id, name FROM users WHERE age >= 40")
+        val plan = db.explain("EXPLAIN SELECT * FROM adults")
+        assertTrue(plan.any { it == "Seq Scan on adults" })
+    }
+
+    @Test
+    fun explainOverADerivedTableDescribesItByItsAlias() {
+        val db = newDatabase()
+        db.seedUsers()
+        val plan = db.explain("EXPLAIN SELECT s.name FROM (SELECT id, name FROM users) AS s")
+        assertTrue(plan.any { it == "Seq Scan on s" })
+    }
+
+    @Test
+    fun explainRejectsAWindowFunctionInWhereStructurallyWithoutRunningTheQuery() {
+        val db = newDatabase()
+        db.seedUsers()
+        val table = db.catalog.table("users")
+        val failure = assertFailsWith<DiskExecutorException> {
+            db.explain("EXPLAIN SELECT name FROM users WHERE ROW_NUMBER() OVER (ORDER BY id) > 1")
+        }
+        assertTrue(failure.message!!.contains("window functions are not allowed in WHERE"))
+        assertEquals(0, table.scanCount)
+    }
+
+    @Test
+    fun explainRejectsAColumnMissingFromGroupByJustLikeRunningTheQueryWould() {
+        val db = newDatabase()
+        db.seedOrders()
+        val failure = assertFailsWith<DiskExecutorException> {
+            db.explain("EXPLAIN SELECT customer_id, amount FROM orders GROUP BY customer_id")
+        }
+        assertTrue(failure.message!!.contains("must appear in GROUP BY"))
+    }
+
+    @Test
+    fun explainRejectsADuplicateTableAliasJustLikeRunningTheQueryWould() {
+        val db = newDatabase()
+        db.seedUsers()
+        val failure = assertFailsWith<DiskExecutorException> {
+            db.explain("EXPLAIN SELECT * FROM users u JOIN users u ON u.id = u.id")
+        }
+        assertTrue(failure.message!!.contains("duplicate table alias"))
+    }
+
+    @Test
+    fun explainOnAMissingTableStillReportsNoSuchTable() {
+        val db = newDatabase()
+        assertFailsWith<DiskCatalogException> { db.explain("EXPLAIN SELECT * FROM missing") }
+    }
+}