Choose minimal covering index for MIN/MAX and DISTINCT

h2/src/main/org/h2/command/dml/Select.java

@@ -889,7 +889,7 @@ public class Select extends Query {
                 Column column = ((ExpressionColumn) expr).getColumn();
                 int selectivity = column.getSelectivity();
                 Index columnIndex = topTableFilter.getTable().
-                        getIndexForColumn(column);
+                        getIndexForColumn(column, false, true);
                 if (columnIndex != null &&
                         selectivity != Constants.SELECTIVITY_DEFAULT &&
                         selectivity < 20) {

h2/src/main/org/h2/expression/Aggregate.java

@@ -287,7 +287,7 @@ public class Aggregate extends Expression {
             case MIN:
             case MAX:
                 boolean first = type == MIN;
-                Index index = getColumnIndex();
+                Index index = getMinMaxColumnIndex();
                 int sortType = index.getIndexColumns()[0].sortType;
                 if ((sortType & SortOrder.DESCENDING) != 0) {
                     first = !first;
@@ -575,14 +575,14 @@ public class Aggregate extends Expression {
         return text + StringUtils.enclose(on.getSQL());
     }
 
-    private Index getColumnIndex() {
+    private Index getMinMaxColumnIndex() {
         if (on instanceof ExpressionColumn) {
             ExpressionColumn col = (ExpressionColumn) on;
             Column column = col.getColumn();
             TableFilter filter = col.getTableFilter();
             if (filter != null) {
                 Table table = filter.getTable();
-                Index index = table.getIndexForColumn(column);
+                Index index = table.getIndexForColumn(column, true, false);
                 return index;
             }
         }
@@ -602,7 +602,7 @@ public class Aggregate extends Expression {
                 return visitor.getTable().canGetRowCount();
             case MIN:
             case MAX:
-                Index index = getColumnIndex();
+                Index index = getMinMaxColumnIndex();
                 return index != null;
             default:
                 return false;

h2/src/main/org/h2/table/Table.java

@@ -1050,22 +1050,35 @@ public abstract class Table extends SchemaObjectBase {
      * This method returns null if no matching index is found.
      *
      * @param column the column
+     * @param needGetFirstOrLast if the returned index must be able
+     *          to do {@link Index#canGetFirstOrLast()}
+     * @param needFindNext if the returned index must be able to do
+     *          {@link Index#findNext(Session, SearchRow, SearchRow)}
      * @return the index or null
      */
-    public Index getIndexForColumn(Column column) {
+    public Index getIndexForColumn(Column column,
+            boolean needGetFirstOrLast, boolean needFindNext) {
         ArrayList<Index> indexes = getIndexes();
+        Index result = null;
         if (indexes != null) {
             for (int i = 1, size = indexes.size(); i < size; i++) {
                 Index index = indexes.get(i);
-                if (index.canGetFirstOrLast()) {
-                    int idx = index.getColumnIndex(column);
-                    if (idx == 0) {
-                        return index;
-                    }
+                if (needGetFirstOrLast && !index.canGetFirstOrLast()) {
+                    continue;
+                }
+                if (needFindNext && !index.canFindNext()) {
+                    continue;
+                }
+                int idx = index.getColumnIndex(column);
+                // choose the minimal covering index with the needed first column
+                // to work consistently with execution plan from Optimizer
+                if (idx == 0 && (result == null ||
+                        result.getColumns().length > index.getColumns().length)) {
+                    result = index;
                 }
             }
         }
-        return null;
+        return result;
     }
 
     public boolean getOnCommitDrop() {