new status variable (number of queries deleted because of low memory)

mysql-test/r/query_cache.result:
  test of new status variable
mysql-test/t/query_cache.test:
  test of new status variable

mysql-test/r/query_cache.result

@@ -320,6 +320,9 @@ insert into t1 select * from t2;
 show status like "Qcache_hits";
 Variable_name	Value
 Qcache_hits	4
+show status like "Qcache_lowmem_prunes";
+Variable_name	Value
+Qcache_lowmem_prunes	0
 select a as a1, a as a2 from t1;
 select a as a2, a as a3 from t1;
 select a as a3, a as a4 from t1;
@@ -330,6 +333,9 @@ Qcache_hits	4
 show status like "Qcache_queries_in_cache";
 Variable_name	Value
 Qcache_queries_in_cache	2
+show status like "Qcache_lowmem_prunes";
+Variable_name	Value
+Qcache_lowmem_prunes	2
 reset query cache;
 insert into t2 select * from t1;
 insert into t1 select * from t2;

mysql-test/t/query_cache.test

@@ -206,6 +206,7 @@ insert into t2 select * from t1; # 2584
 insert into t1 select * from t2; # 4181
 
 show status like "Qcache_hits";
+show status like "Qcache_lowmem_prunes";
 disable_result_log;
 select a as a1, a as a2 from t1;
 select a as a2, a as a3 from t1;
@@ -215,6 +216,7 @@ select a as a1, a as a2 from t1;
 enable_result_log;
 show status like "Qcache_hits";
 show status like "Qcache_queries_in_cache";
+show status like "Qcache_lowmem_prunes";
 reset query cache;
 #
 # Query bigger then query_cache_limit

sql/mysqld.cc

@@ -3793,6 +3793,7 @@ struct show_var_st status_vars[]= {
   {"Qcache_queries_in_cache",  (char*) &query_cache.queries_in_cache, SHOW_LONG_CONST},
   {"Qcache_inserts",           (char*) &query_cache.inserts,    SHOW_LONG},
   {"Qcache_hits",              (char*) &query_cache.hits,       SHOW_LONG},
+  {"Qcache_lowmem_prunes",     (char*) &query_cache.lowmem_prunes, SHOW_LONG},
   {"Qcache_not_cached",        (char*) &query_cache.refused,    SHOW_LONG},
   {"Qcache_free_memory",       (char*) &query_cache.free_memory, 
    SHOW_LONG_CONST},

sql/sql_cache.cc

@@ -708,7 +708,7 @@ Query_cache::Query_cache(ulong query_cache_limit_arg,
   :query_cache_size(0),
    query_cache_limit(query_cache_limit_arg),
    queries_in_cache(0), hits(0), inserts(0), refused(0),
-   total_blocks(0),
+   total_blocks(0), lowmem_prunes(0),
    min_allocation_unit(ALIGN_SIZE(min_allocation_unit_arg)),
    min_result_data_size(ALIGN_SIZE(min_result_data_size_arg)),
    def_query_hash_size(ALIGN_SIZE(def_query_hash_size_arg)),
@@ -1512,6 +1512,7 @@ my_bool Query_cache::free_old_query()
     if (query_block != 0)
     {
       free_query(query_block);
+      lowmem_prunes++;
       DBUG_RETURN(0);
     }
   }

sql/sql_cache.h

@@ -219,7 +219,7 @@ public:
   ulong query_cache_size, query_cache_limit;
   /* statistics */
   ulong free_memory, queries_in_cache, hits, inserts, refused,
-    free_memory_blocks, total_blocks;
+    free_memory_blocks, total_blocks, lowmem_prunes;
 
 protected:
   /*