EXPLAIN ANALYZE now also lists the number of pages read from the file.

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

@@ -6,12 +6,16 @@
  */
 package org.h2.command.dml;
 
+import java.util.Map;
+import java.util.TreeMap;
+import java.util.Map.Entry;
 import org.h2.command.Prepared;
 import org.h2.engine.Session;
 import org.h2.expression.Expression;
 import org.h2.expression.ExpressionColumn;
 import org.h2.result.LocalResult;
 import org.h2.result.ResultInterface;
+import org.h2.store.PageStore;
 import org.h2.table.Column;
 import org.h2.value.Value;
 import org.h2.value.ValueString;
@@ -52,14 +56,37 @@ public class Explain extends Prepared {
         Expression[] expressions = { expr };
         result = new LocalResult(session, expressions, 1);
         if (maxrows >= 0) {
+            String plan;
             if (executeCommand) {
+                PageStore store = session.getDatabase().getPageStore();
+                store.statisticsStart();
                 if (command.isQuery()) {
                     command.query(maxrows);
                 } else {
                     command.update();
                 }
+                plan = command.getPlanSQL();
+                Map<String, Integer> statistics = store.statisticsEnd();
+                if (statistics != null) {
+                    int total = 0;
+                    for (Entry<String, Integer> e : statistics.entrySet()) {
+                        total += e.getValue();
+                    }
+                    if (total > 0) {
+                        statistics = new TreeMap<String, Integer>(statistics);
+                        StringBuilder buff = new StringBuilder();
+                        buff.append("total: ").append(total).append('\n');
+                        for (Entry<String, Integer> e : statistics.entrySet()) {
+                            int value = e.getValue();
+                            int percent = (int) (100L * value / total);
+                            buff.append(e.getKey()).append(": ").append(value).append(" (").append(percent).append("%)\n");
+                        }
+                        plan += "\n/*\n" + buff.toString() + "*/";
+                    }
+                }
+            } else {
+                plan = command.getPlanSQL();
             }
-            String plan = command.getPlanSQL();
             add(plan);
         }
         result.done();

h2/src/main/org/h2/store/PageStore.java

@@ -205,6 +205,8 @@ public class PageStore implements CacheWriter {
 
     private long logSizeBase;
 
+    private HashMap<String, Integer> statistics;
+
     /**
      * Create a new page store object.
      *
@@ -225,6 +227,31 @@ public class PageStore implements CacheWriter {
         systemSession = new Session(database, null, 0);
     }
 
+    /**
+     * Start collecting statistics.
+     */
+    public void statisticsStart() {
+        statistics = New.hashMap();
+    }
+
+    /**
+     * Stop collecting statistics.
+     *
+     * @return the statistics
+     */
+    public HashMap<String, Integer> statisticsEnd() {
+        HashMap<String, Integer> result = statistics;
+        statistics = null;
+        return result;
+    }
+
+    private void statisticsIncrement(String key) {
+        if (statistics != null) {
+            Integer old = statistics.get(key);
+            statistics.put(key, old == null ? 1 : old + 1);
+        }
+    }
+
     /**
      * Copy the next page to the output stream.
      *
@@ -546,6 +573,9 @@ public class PageStore implements CacheWriter {
                 if (index == null) {
                     throw DbException.get(ErrorCode.FILE_CORRUPTED_1, "index not found " + indexId);
                 }
+                if (statistics != null) {
+                    statisticsIncrement(index.getTable().getName() + "." + index.getName() + " read");
+                }
                 p = PageDataLeaf.read(index, data, pageId);
                 break;
             }
@@ -555,11 +585,17 @@ public class PageStore implements CacheWriter {
                 if (index == null) {
                     throw DbException.get(ErrorCode.FILE_CORRUPTED_1, "index not found " + indexId);
                 }
+                if (statistics != null) {
+                    statisticsIncrement(index.getTable().getName() + "." + index.getName() + " read");
+                }
                 p = PageDataNode.read(index, data, pageId);
                 break;
             }
             case Page.TYPE_DATA_OVERFLOW: {
                 p = PageDataOverflow.read(this, data, pageId);
+                if (statistics != null) {
+                    statisticsIncrement("overflow read");
+                }
                 break;
             }
             case Page.TYPE_BTREE_LEAF: {
@@ -568,6 +604,9 @@ public class PageStore implements CacheWriter {
                 if (index == null) {
                     throw DbException.get(ErrorCode.FILE_CORRUPTED_1, "index not found " + indexId);
                 }
+                if (statistics != null) {
+                    statisticsIncrement(index.getTable().getName() + "." + index.getName() + " read");
+                }
                 p = PageBtreeLeaf.read(index, data, pageId);
                 break;
             }
@@ -577,6 +616,9 @@ public class PageStore implements CacheWriter {
                 if (index == null) {
                     throw DbException.get(ErrorCode.FILE_CORRUPTED_1, "index not found " + indexId);
                 }
+                if (statistics != null) {
+                    statisticsIncrement(index.getTable().getName() + "." + index.getName() + " read");
+                }
                 p = PageBtreeNode.read(index, data, pageId);
                 break;
             }