Documentation.

h2/src/docsrc/html/performance.html

@@ -678,6 +678,10 @@ total: 85
 TEST.TEST_DATA read: 85 (100%)
 */
 </pre>
+<p>
+The cache will prevent the pages are read twice. H2 reads all columns of the row 
+unless only the columns in the index are read. Except for large CLOB and BLOB, which are not store in the table. 
+</p>
 
 <h3>Special Optimizations</h3>
 <p>