branches/zip:

Limit the number of the pages that are sampled so it is never greater
than the total number of pages in the index.

The parameter that specifies the number of pages to test is global for
all tables. By limiting it this way we allow the user to set it "high"
to suit "large" tables and to avoid unnecessary work for "small" tables
(e.g. doing 100 dives in a table that has 5 pages, obviously testing
some pages more than once).

Suggested by:	Ken
Approved by:	Marko

btr/btr0cur.c

@@ -3151,6 +3151,7 @@ btr_estimate_number_of_different_key_vals(
 	ulint		matched_fields;
 	ulint		matched_bytes;
 	ib_int64_t*	n_diff;
+	ullint		n_sample_pages; /* number of pages to sample */
 	ulint		not_empty_flag	= 0;
 	ulint		total_external_size = 0;
 	ulint		i;
@@ -3169,9 +3170,21 @@ btr_estimate_number_of_different_key_vals(
 
 	n_diff = mem_zalloc((n_cols + 1) * sizeof(ib_int64_t));
 
+	/* It makes no sense to test more pages than are contained
+	in the index, thus we lower the number if it is too high */
+	if (srv_stats_sample_pages > index->stat_index_size) {
+		if (index->stat_index_size > 0) {
+			n_sample_pages = index->stat_index_size;
+		} else {
+			n_sample_pages = 1;
+		}
+	} else {
+		n_sample_pages = srv_stats_sample_pages;
+	}
+
 	/* We sample some pages in the index to get an estimate */
 
-	for (i = 0; i < srv_stats_sample_pages; i++) {
+	for (i = 0; i < n_sample_pages; i++) {
 		rec_t*	supremum;
 		mtr_start(&mtr);
 
@@ -3260,7 +3273,7 @@ btr_estimate_number_of_different_key_vals(
 	}
 
 	/* If we saw k borders between different key values on
-	srv_stats_sample_pages leaf pages, we can estimate how many
+	n_sample_pages leaf pages, we can estimate how many
 	there will be in index->stat_n_leaf_pages */
 
 	/* We must take into account that our sample actually represents
@@ -3271,26 +3284,26 @@ btr_estimate_number_of_different_key_vals(
 		index->stat_n_diff_key_vals[j]
 			= ((n_diff[j]
 			    * (ib_int64_t)index->stat_n_leaf_pages
-			    + srv_stats_sample_pages - 1
+			    + n_sample_pages - 1
 			    + total_external_size
 			    + not_empty_flag)
-			   / (srv_stats_sample_pages
+			   / (n_sample_pages
 			      + total_external_size));
 
 		/* If the tree is small, smaller than
-		10 * srv_stats_sample_pages + total_external_size, then
+		10 * n_sample_pages + total_external_size, then
 		the above estimate is ok. For bigger trees it is common that we
 		do not see any borders between key values in the few pages
-		we pick. But still there may be srv_stats_sample_pages
+		we pick. But still there may be n_sample_pages
 		different key values, or even more. Let us try to approximate
 		that: */
 
 		add_on = index->stat_n_leaf_pages
-			/ (10 * (srv_stats_sample_pages
+			/ (10 * (n_sample_pages
 				 + total_external_size));
 
-		if (add_on > srv_stats_sample_pages) {
-			add_on = srv_stats_sample_pages;
+		if (add_on > n_sample_pages) {
+			add_on = n_sample_pages;
 		}
 
 		index->stat_n_diff_key_vals[j] += add_on;