Add a way for virtual tables to return the expected number of rows for a scan (not just the overall cost) to SQLite. Have the rtree module make use of this.

FossilOrigin-Name: 5a3cfd747a85480d215784817c3821d87ecfa2f7

src/sqlite.h.in

@@ -5290,10 +5290,22 @@ struct sqlite3_module {
 ** the correct order to satisfy the ORDER BY clause so that no separate
 ** sorting step is required.
 **
-** ^The estimatedCost value is an estimate of the cost of doing the
-** particular lookup.  A full scan of a table with N entries should have
-** a cost of N.  A binary search of a table of N entries should have a
-** cost of approximately log(N).
+** ^The estimatedCost value is an estimate of the cost of a particular
+** strategy. A cost of N indicates that the cost of the strategy is similar
+** to a linear scan of an SQLite table with N rows. A cost of log(N) 
+** indicates that the expense of the operation is similar to that of a
+** binary search on a unique indexed field of an SQLite table with N rows.
+**
+** ^The estimatedRows value is an estimate of the number of rows that
+** will be returned by the strategy.
+**
+** IMPORTANT: The estimatedRows field was added to the sqlite3_index_info
+** structure for SQLite version 3.8.2. If a virtual table extension is
+** used with an SQLite version earlier than 3.8.2, the results of attempting 
+** to read or write the estimatedRows field are undefined (but are likely 
+** to included crashing the application). The estimatedRows field should
+** therefore only be used if [sqlite3_libversion_number()] returns a
+** value greater than or equal to 3008002.
 */
 struct sqlite3_index_info {
   /* Inputs */
@@ -5318,7 +5330,8 @@ struct sqlite3_index_info {
   char *idxStr;              /* String, possibly obtained from sqlite3_malloc */
   int needToFreeIdxStr;      /* Free idxStr using sqlite3_free() if true */
   int orderByConsumed;       /* True if output is already ordered */
-  double estimatedCost;      /* Estimated cost of using this index */
+  double estimatedCost;           /* Estimated cost of using this index */
+  sqlite3_int64 estimatedRows;    /* Estimated number of rows returned */
 };
 
 /*