Enable SELECT query planning tracing when compiled with

SQLITE_ENABLE_SELECTTRACE and either SQLITE_DEBUG or SQLITE_TEST.

FossilOrigin-Name: cbe0cf9ddf46f0a678c85d49bfa74e3b7712e1a1

src/select.c

@@ -14,6 +14,17 @@
 */
 #include "sqliteInt.h"
 
+/*
+** Trace output macros
+*/
+#if SELECTTRACE_ENABLED
+/***/ int sqlite3SelectTrace = 0;
+# define SELECTTRACE(K,X)  if(sqlite3SelectTrace&(K)) sqlite3DebugPrintf X
+#else
+# define SELECTTRACE(K,X)
+#endif
+
+
 /*
 ** An instance of the following object is used to record information about
 ** how to process the DISTINCT keyword, to simplify passing that information
@@ -3355,6 +3366,8 @@ static int flattenSubquery(
   }
 
   /***** If we reach this point, flattening is permitted. *****/
+  SELECTTRACE(1, ("flatten %s (term %d) into %s\n",
+                   pSub->zSelLabel, iFrom, p->zSelLabel));
 
   /* Authorize the subquery */
   pParse->zAuthContext = pSubitem->zName;