The MERGE statement is now about 30% faster when using a PreparedStatement.

h2/src/docsrc/html/changelog.html

@@ -18,7 +18,8 @@ Change Log
 <h1>Change Log</h1>
 
 <h2>Next Version (unreleased)</h2>
-<ul><li>Multi-column indexes where the second or later column was descending did not always
+<ul><li>The MERGE statement is now about 30% faster when using a PreparedStatement.
+</li><li>Multi-column indexes where the second or later column was descending did not always
     produce correct results (rows were missing in the result set, or the result set was empty).
 </li><li>When using large transactions or a small log size, the database could get very slow
     (profiling shows the hotspot is in FileObjectDisk.sync()).

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

@@ -80,27 +80,6 @@ public class Merge extends Prepared {
         int count;
         session.getUser().checkRight(table, Right.INSERT);
         session.getUser().checkRight(table, Right.UPDATE);
-        if (keys == null) {
-            Index idx = table.getPrimaryKey();
-            if (idx == null) {
-                throw DbException.get(ErrorCode.CONSTRAINT_NOT_FOUND_1, "PRIMARY KEY");
-            }
-            keys = idx.getColumns();
-        }
-        StatementBuilder buff = new StatementBuilder("UPDATE ");
-        buff.append(table.getSQL()).append(" SET ");
-        for (Column c : columns) {
-            buff.appendExceptFirst(", ");
-            buff.append(c.getSQL()).append("=?");
-        }
-        buff.append(" WHERE ");
-        buff.resetCount();
-        for (Column c : keys) {
-            buff.appendExceptFirst(" AND ");
-            buff.append(c.getSQL()).append("=?");
-        }
-        String sql = buff.toString();
-        update = session.prepare(sql);
         setCurrentRowNumber(0);
         session.setLastIdentity(ValueLong.get(0));
         if (list.size() > 0) {
@@ -263,6 +242,27 @@ public class Merge extends Prepared {
                 throw DbException.get(ErrorCode.COLUMN_COUNT_DOES_NOT_MATCH);
             }
         }
+        if (keys == null) {
+            Index idx = table.getPrimaryKey();
+            if (idx == null) {
+                throw DbException.get(ErrorCode.CONSTRAINT_NOT_FOUND_1, "PRIMARY KEY");
+            }
+            keys = idx.getColumns();
+        }
+        StatementBuilder buff = new StatementBuilder("UPDATE ");
+        buff.append(table.getSQL()).append(" SET ");
+        for (Column c : columns) {
+            buff.appendExceptFirst(", ");
+            buff.append(c.getSQL()).append("=?");
+        }
+        buff.append(" WHERE ");
+        buff.resetCount();
+        for (Column c : keys) {
+            buff.appendExceptFirst(" AND ");
+            buff.append(c.getSQL()).append("=?");
+        }
+        String sql = buff.toString();
+        update = session.prepare(sql);
     }
 
     public boolean isTransactional() {

h2/src/test/org/h2/test/test-1.2.txt

@@ -4322,8 +4322,8 @@ EXPLAIN SELECT * FROM TEST WHERE ID=1;
 
 EXPLAIN MERGE INTO TEST VALUES(1, 'Hello');
 > PLAN
-> ----------------------------------------------------
-> MERGE INTO PUBLIC.TEST(ID, NAME) VALUES (1, 'Hello')
+> ------------------------------------------------------------
+> MERGE INTO PUBLIC.TEST(ID, NAME) KEY(ID) VALUES (1, 'Hello')
 > rows: 1
 
 MERGE INTO TEST VALUES(1, 'Hello');
@@ -4343,8 +4343,8 @@ MERGE INTO TEST(ID, NAME) VALUES(3, 'How are you');
 
 EXPLAIN MERGE INTO TEST(ID, NAME) VALUES(3, 'How are you');
 > PLAN
-> ----------------------------------------------------------
-> MERGE INTO PUBLIC.TEST(ID, NAME) VALUES (3, 'How are you')
+> ------------------------------------------------------------------
+> MERGE INTO PUBLIC.TEST(ID, NAME) KEY(ID) VALUES (3, 'How are you')
 > rows: 1
 
 MERGE INTO TEST(ID, NAME) KEY(ID) VALUES(3, 'How do you do');