perf: skip caching of very large queries to prevent statement cache pollution

This serves as a basic protection from cache pollution. More sophisticated one will come in pgjdbc#344

Author: vlsi

doc/pgjdbc.xml

@@ -766,6 +766,8 @@ openssl pkcs8 -topk8 -in client.key -out client.pk8 -outform DER -v1 PBE-SHA1-3D
          the least recently used ones will be discarded.
          The main aim of this setting is to prevent <classname>OutOfMemoryError</classname>.
          The value of 0 disables the cache.
+         If a query would consume more than a half of <varname>preparedStatementCacheSizeMiB</varname>,
+         then it is discarded immediately.
         </para>
        </listitem>
       </varlistentry>

org/postgresql/util/LruCache.java

@@ -103,12 +103,13 @@ public Value borrow(Key key) throws SQLException
      */
     public void put(Key key, Value value)
     {
-        if (maxSizeBytes == 0 || maxSizeEntries == 0) {
-            // Just destroy the value if cache is disabled
+        long valueSize = value.getSize();
+        if (maxSizeBytes == 0 || maxSizeEntries == 0 || valueSize * 2 > maxSizeBytes) {
+            // Just destroy the value if cache is disabled or if entry would consume more than a half of the cache
             evictValue(value);
             return;
         }
-        currentSize += value.getSize();
+        currentSize += valueSize;
         cache.put(key, value);
     }
 }