Merge pull request #1589 from katzyn/cost

Improve cost calculation for multi-column indexes

h2/src/main/org/h2/index/BaseIndex.java

@@ -168,12 +168,14 @@ public abstract class BaseIndex extends SchemaObjectBase implements Index {
         int totalSelectivity = 0;
         long rowsCost = rowCount;
         if (masks != null) {
-            for (int i = 0, len = columns.length; i < len; i++) {
-                Column column = columns[i];
+            int i = 0, len = columns.length;
+            boolean tryAdditional = false;
+            while (i < len) {
+                Column column = columns[i++];
                 int index = column.getColumnId();
                 int mask = masks[index];
                 if ((mask & IndexCondition.EQUALITY) == IndexCondition.EQUALITY) {
-                    if (i == columns.length - 1 && getIndexType().isUnique()) {
+                    if (i == len && getIndexType().isUnique()) {
                         rowsCost = 3;
                         break;
                     }
@@ -185,18 +187,34 @@ public abstract class BaseIndex extends SchemaObjectBase implements Index {
                     }
                     rowsCost = 2 + Math.max(rowCount / distinctRows, 1);
                 } else if ((mask & IndexCondition.RANGE) == IndexCondition.RANGE) {
-                    rowsCost = 2 + rowCount / 4;
+                    rowsCost = 2 + rowsCost / 4;
+                    tryAdditional = true;
                     break;
                 } else if ((mask & IndexCondition.START) == IndexCondition.START) {
-                    rowsCost = 2 + rowCount / 3;
+                    rowsCost = 2 + rowsCost / 3;
+                    tryAdditional = true;
                     break;
                 } else if ((mask & IndexCondition.END) == IndexCondition.END) {
-                    rowsCost = rowCount / 3;
+                    rowsCost = rowsCost / 3;
+                    tryAdditional = true;
                     break;
                 } else {
+                    if (mask == 0) {
+                        // Adjust counter of used columns (i)
+                        i--;
+                    }
                     break;
                 }
             }
+            // Some additional columns can still be used
+            if (tryAdditional) {
+                while (i < len && masks[columns[i].getColumnId()] != 0) {
+                    i++;
+                    rowsCost--;
+                }
+            }
+            // Increase cost of indexes with additional unused columns
+            rowsCost += len - i;
         }
         // If the ORDER BY clause matches the ordering of this index,
         // it will be cheaper than another index, so adjust the cost

h2/src/test/org/h2/test/scripts/indexes.sql

@@ -304,3 +304,49 @@ select 1 from test group by x;
 
 drop table test;
 > ok
+
+CREATE TABLE TEST(A INT, B INT, C INT);
+> ok
+
+CREATE INDEX T_A1 ON TEST(A);
+> ok
+
+CREATE INDEX T_A_B ON TEST(A, B);
+> ok
+
+CREATE INDEX T_A_C ON TEST(A, C);
+> ok
+
+EXPLAIN SELECT * FROM TEST WHERE A = 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A1: A = 0 */ WHERE A = 0
+
+EXPLAIN SELECT * FROM TEST WHERE A = 0 AND B >= 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A_B: A = 0 AND B >= 0 */ WHERE (A = 0) AND (B >= 0)
+
+EXPLAIN SELECT * FROM TEST WHERE A > 0 AND B >= 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A_B: A > 0 AND B >= 0 */ WHERE (A > 0) AND (B >= 0)
+
+INSERT INTO TEST (SELECT X / 100, X, X FROM SYSTEM_RANGE(1, 3000));
+> update count: 3000
+
+EXPLAIN SELECT * FROM TEST WHERE A = 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A1: A = 0 */ WHERE A = 0
+
+EXPLAIN SELECT * FROM TEST WHERE A = 0 AND B >= 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A_B: A = 0 AND B >= 0 */ WHERE (A = 0) AND (B >= 0)
+
+EXPLAIN SELECT * FROM TEST WHERE A > 0 AND B >= 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A_B: A > 0 AND B >= 0 */ WHERE (A > 0) AND (B >= 0)
+
+-- Test that creation order of indexes has no effect
+CREATE INDEX T_A2 ON TEST(A);
+> ok
+
+DROP INDEX T_A1;
+> ok
+
+EXPLAIN SELECT * FROM TEST WHERE A = 0;
+>> SELECT TEST.A, TEST.B, TEST.C FROM PUBLIC.TEST /* PUBLIC.T_A2: A = 0 */ WHERE A = 0
+
+DROP TABLE TEST;
+> ok