Adds support for DISTINCT in Eval

partiql-eval/src/main/kotlin/org/partiql/eval/internal/Compiler.kt

@@ -1,6 +1,7 @@
 package org.partiql.eval.internal
 
 import org.partiql.eval.internal.operator.Operator
+import org.partiql.eval.internal.operator.rel.RelDistinct
 import org.partiql.eval.internal.operator.rel.RelFilter
 import org.partiql.eval.internal.operator.rel.RelJoinInner
 import org.partiql.eval.internal.operator.rel.RelJoinLeft
@@ -174,6 +175,11 @@ internal class Compiler(
         return ExprLiteral(node.value)
     }
 
+    override fun visitRelOpDistinct(node: Rel.Op.Distinct, ctx: Unit): Operator {
+        val input = visitRel(node.input, ctx)
+        return RelDistinct(input)
+    }
+
     override fun visitRelOpFilter(node: Rel.Op.Filter, ctx: Unit): Operator {
         val input = visitRel(node.input, ctx)
         val condition = visitRex(node.predicate, ctx)

partiql-eval/src/main/kotlin/org/partiql/eval/internal/operator/rel/RelDistinct.kt

@@ -0,0 +1,31 @@
+package org.partiql.eval.internal.operator.rel
+
+import org.partiql.eval.internal.Record
+import org.partiql.eval.internal.operator.Operator
+
+internal class RelDistinct(
+    val input: Operator.Relation
+) : Operator.Relation {
+
+    private val seen = mutableSetOf<Record>()
+
+    override fun open() {
+        input.open()
+    }
+
+    override fun next(): Record? {
+        var next = input.next()
+        while (next != null) {
+            if (seen.contains(next).not()) {
+                seen.add(next)
+                return next
+            }
+            next = input.next()
+        }
+        return null
+    }
+
+    override fun close() {
+        input.close()
+    }
+}