diff --git a/src/test/kotlin/kestrel/store/sql/DiskExecutorDistinctTest.kt b/src/test/kotlin/kestrel/store/sql/DiskExecutorDistinctTest.kt
new file mode 100644
index 0000000..7a2a495
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+++ b/src/test/kotlin/kestrel/store/sql/DiskExecutorDistinctTest.kt
@@ -0,0 +1,167 @@
+package kestrel.store.sql
+
+import kestrel.engine.SelectResult
+import kestrel.engine.StatementResult
+import kestrel.sql.lexer.Lexer
+import kestrel.sql.parser.Parser
+import kestrel.storage.Value
+import java.io.File
+import java.nio.file.Files
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+
+/**
+ * `SELECT DISTINCT`/`SELECT DISTINCT ON (...)` through [DiskExecutor] against a real [DiskCatalog]
+ * backed by temp files - ports [kestrel.DistinctTest]'s in-memory scenarios 1:1, since `runSelect`
+ * shares the same dedup pipeline shape as [kestrel.engine.SelectRunner]'s `Plan.projectRows`.
+ * [distinctSurvivesClosingAndReopeningTheCatalog] has no in-memory equivalent - it exercises the one
+ * thing only the disk-backed engine's persistence layer can demonstrate.
+ */
+class DiskExecutorDistinctTest {
+    private fun tempFile(prefix: String): File = Files.createTempFile(prefix, ".db").toFile().also { it.deleteOnExit() }
+
+    private fun int(value: Long) = Value.IntegerValue(value)
+
+    private fun text(value: String) = Value.TextValue(value)
+
+    private class Database(val dataFile: File, val walFile: File) {
+        var catalog = DiskCatalog.open(dataFile, walFile)
+            private set
+        var executor = DiskExecutor(catalog)
+            private set
+
+        fun run(sql: String): List<StatementResult> = Parser(Lexer(sql).tokenize()).parseProgram().map { executor.execute(it) }
+        fun exec(sql: String): StatementResult = run(sql).single()
+        fun select(sql: String): SelectResult = exec(sql) as SelectResult
+
+        fun reopen() {
+            catalog.close()
+            catalog = DiskCatalog.open(dataFile, walFile)
+            executor = DiskExecutor(catalog)
+        }
+    }
+
+    private fun newDatabase(): Database = Database(tempFile("kestrel-disk-distinct-data"), tempFile("kestrel-disk-distinct-wal"))
+
+    private fun Database.seedSales() {
+        run("CREATE TABLE sales (dept TEXT, region TEXT, amount INTEGER)")
+        run(
+            "INSERT INTO sales VALUES " +
+                "('eng', 'eu', 100), ('eng', 'us', 200), ('eng', 'eu', 100), " +
+                "('ops', 'eu', 50), ('ops', 'eu', 70), ('ops', 'eu', 50)"
+        )
+    }
+
+    @Test
+    fun distinctRemovesDuplicateOutputRows() {
+        val db = newDatabase()
+        db.seedSales()
+        val result = db.select("SELECT DISTINCT dept FROM sales ORDER BY dept")
+        assertEquals(listOf(listOf(text("eng")), listOf(text("ops"))), result.rows)
+    }
+
+    @Test
+    fun distinctStarDedupsOnEveryColumn() {
+        val db = newDatabase()
+        db.seedSales()
+        val result = db.select("SELECT DISTINCT * FROM sales ORDER BY dept, region, amount")
+        assertEquals(4, result.rows.size)
+    }
+
+    @Test
+    fun distinctWithLimitCountsDistinctRowsNotSourceRows() {
+        val db = newDatabase()
+        db.seedSales()
+        val result = db.select("SELECT DISTINCT dept FROM sales ORDER BY dept LIMIT 1")
+        assertEquals(listOf(listOf(text("eng"))), result.rows)
+    }
+
+    @Test
+    fun distinctWithOffsetSkipsDistinctRowsNotSourceRows() {
+        val db = newDatabase()
+        db.seedSales()
+        val result = db.select("SELECT DISTINCT dept FROM sales ORDER BY dept LIMIT 1 OFFSET 1")
+        assertEquals(listOf(listOf(text("ops"))), result.rows)
+    }
+
+    @Test
+    fun distinctCombinesWithGroupBy() {
+        val db = newDatabase()
+        db.run("CREATE TABLE t (a INTEGER, b INTEGER)")
+        db.run("INSERT INTO t VALUES (1, 10), (1, 20), (2, 10)")
+        val result = db.select("SELECT DISTINCT a FROM t GROUP BY a, b ORDER BY a")
+        assertEquals(listOf(listOf(int(1)), listOf(int(2))), result.rows)
+    }
+
+    @Test
+    fun distinctOnKeepsFirstRowPerGroupInOrderByOrder() {
+        val db = newDatabase()
+        db.seedSales()
+        val result = db.select("SELECT DISTINCT ON (dept) dept, amount FROM sales ORDER BY dept, amount DESC")
+        assertEquals(listOf(listOf(text("eng"), int(200)), listOf(text("ops"), int(70))), result.rows)
+    }
+
+    @Test
+    fun distinctOnWithoutOrderByUsesScanOrder() {
+        val db = newDatabase()
+        db.run("CREATE TABLE t (k TEXT, v INTEGER)")
+        db.run("INSERT INTO t VALUES ('a', 1), ('a', 2), ('b', 3)")
+        val result = db.select("SELECT DISTINCT ON (k) k, v FROM t")
+        assertEquals(listOf(listOf(text("a"), int(1)), listOf(text("b"), int(3))), result.rows)
+    }
+
+    @Test
+    fun distinctOnRequiresOrderByToStartWithItsOwnExpressions() {
+        val db = newDatabase()
+        db.seedSales()
+        assertFailsWith<DiskExecutorException> {
+            db.select("SELECT DISTINCT ON (dept) dept, amount FROM sales ORDER BY amount")
+        }
+    }
+
+    @Test
+    fun distinctOnRejectsGroupBy() {
+        val db = newDatabase()
+        db.seedSales()
+        assertFailsWith<DiskExecutorException> {
+            db.select("SELECT DISTINCT ON (dept) dept, SUM(amount) FROM sales GROUP BY dept")
+        }
+    }
+
+    @Test
+    fun distinctOnRejectsAWindowFunction() {
+        val db = newDatabase()
+        db.seedSales()
+        assertFailsWith<DiskExecutorException> {
+            db.select("SELECT DISTINCT ON (ROW_NUMBER() OVER (ORDER BY amount)) dept FROM sales")
+        }
+    }
+
+    @Test
+    fun distinctOnRejectsAnUnknownColumn() {
+        val db = newDatabase()
+        db.seedSales()
+        assertFailsWith<DiskExecutorException> {
+            db.select("SELECT DISTINCT ON (nope) dept FROM sales")
+        }
+    }
+
+    @Test
+    fun distinctSurvivesClosingAndReopeningTheCatalog() {
+        val db = newDatabase()
+        db.seedSales()
+        db.reopen()
+        val result = db.select("SELECT DISTINCT dept FROM sales ORDER BY dept")
+        assertEquals(listOf(listOf(text("eng")), listOf(text("ops"))), result.rows)
+    }
+
+    @Test
+    fun distinctOnSurvivesClosingAndReopeningTheCatalog() {
+        val db = newDatabase()
+        db.seedSales()
+        db.reopen()
+        val result = db.select("SELECT DISTINCT ON (dept) dept, amount FROM sales ORDER BY dept, amount DESC")
+        assertEquals(listOf(listOf(text("eng"), int(200)), listOf(text("ops"), int(70))), result.rows)
+    }
+}