Bug#30087 Set query_cache_size, if the value is too small, get a unclear warning

  
This bugs clarifies a warning message issued when the query cache data
size becomes smaller than the minium allowed size.

mysql-test/r/query_cache.result

@@ -564,7 +564,7 @@ select * from t1;
 a
 set GLOBAL query_cache_size=1024;
 Warnings:
-Warning	1282	Query cache failed to set size 1024; new query cache size is 0
+Warning	1282	Query cache failed to set size 1024 (minimal value: 41297); new query cache size is 0
 show global variables like "query_cache_size";
 Variable_name	Value
 query_cache_size	0
@@ -572,7 +572,7 @@ select * from t1;
 a
 set GLOBAL query_cache_size=10240;
 Warnings:
-Warning	1282	Query cache failed to set size 10240; new query cache size is 0
+Warning	1282	Query cache failed to set size 10240 (minimal value: 41297); new query cache size is 0
 show global variables like "query_cache_size";
 Variable_name	Value
 query_cache_size	0
@@ -580,7 +580,7 @@ select * from t1;
 a
 set GLOBAL query_cache_size=20480;
 Warnings:
-Warning	1282	Query cache failed to set size 20480; new query cache size is 0
+Warning	1282	Query cache failed to set size 20480 (minimal value: 41297); new query cache size is 0
 show global variables like "query_cache_size";
 Variable_name	Value
 query_cache_size	0
@@ -588,7 +588,7 @@ select * from t1;
 a
 set GLOBAL query_cache_size=40960;
 Warnings:
-Warning	1282	Query cache failed to set size 40960; new query cache size is 0
+Warning	1282	Query cache failed to set size 40960 (minimal value: 41297); new query cache size is 0
 show global variables like "query_cache_size";
 Variable_name	Value
 query_cache_size	0

sql/set_var.cc

@@ -1342,12 +1342,7 @@ static void fix_net_retry_count(THD *thd __attribute__((unused)),
 static void fix_query_cache_size(THD *thd, enum_var_type type)
 {
 #ifdef HAVE_QUERY_CACHE
-  ulong requested= query_cache_size;
-  query_cache.resize(query_cache_size);
-  if (requested != query_cache_size)
-    push_warning_printf(current_thd, MYSQL_ERROR::WARN_LEVEL_WARN,
-			ER_WARN_QC_RESIZE, ER(ER_WARN_QC_RESIZE),
-			requested, query_cache_size);
+  query_cache_size= query_cache.resize(query_cache_size);
 #endif
 }
 

sql/share/errmsg.txt

@@ -4992,13 +4992,13 @@ ER_WRONG_NAME_FOR_CATALOG 42000
 	spa "Nombre de catalog incorrecto '%-.100s'"
 	swe "Felaktigt katalog namn '%-.100s'"
 ER_WARN_QC_RESIZE  
-	eng "Query cache failed to set size %lu; new query cache size is %lu"
-	ger "nderung der Query-Cache-Gre auf %lu fehlgeschlagen; neue Query-Cache-Gre ist %lu"
-	por "Falha em Query cache para configurar tamanho %lu, novo tamanho de query cache  %lu"
-	rus "      %lu,     - %lu"
-	spa "Query cache fallada para configurar tamao %lu, nuevo tamao de query cache es %lu"
-	swe "Storleken av "Query cache" kunde inte sttas till %lu, ny storlek r %lu"
-	ukr " Ԧ   ͦ %lu,  ͦ  Ԧ - %lu"
+	eng "Query cache failed to set size %lu (minimal value: %lu); new query cache size is %lu"
+	ger "nderung der Query-Cache-Gre auf %lu (Minimale Zahl: %lu) fehlgeschlagen; neue Query-Cache-Gre ist %lu"
+	por "Falha em Query cache para configurar tamanho %lu (Nmero mnimo: %lu), novo tamanho de query cache  %lu"
+	rus "      %lu (minimal value: %lu),     - %lu"
+	spa "Query cache fallada para configurar tamao %lu (Nmero mnimo: %lu), nuevo tamao de query cache es %lu"
+	swe "Storleken av "Query cache" kunde inte sttas till %lu (minsta vrde: %lu); ny storlek r %lu"
+	ukr " Ԧ   ͦ %lu (minimal value: %lu),  ͦ  Ԧ - %lu"
 ER_BAD_FT_COLUMN  
 	eng "Column '%-.64s' cannot be part of FULLTEXT index"
 	ger "Feld '%-.64s' kann nicht Teil eines FULLTEXT-Index sein"

sql/sql_cache.cc

@@ -816,6 +816,16 @@ ulong Query_cache::resize(ulong query_cache_size_arg)
   free_cache();
   query_cache_size= query_cache_size_arg;
   ::query_cache_size= init_cache();
+
+  if (::query_cache_size != query_cache_size_arg)
+  {
+    push_warning_printf(current_thd, MYSQL_ERROR::WARN_LEVEL_WARN,
+                        ER_WARN_QC_RESIZE, ER(ER_WARN_QC_RESIZE),
+                        query_cache_size_arg,
+                        get_minimal_size_limit(),
+                        ::query_cache_size);
+  }
+
   STRUCT_UNLOCK(&structure_guard_mutex);
   DBUG_RETURN(::query_cache_size);
 }
@@ -1614,6 +1624,25 @@ void Query_cache::init()
 }
 
 
+/**
+  Return the lowest possible query cache size.
+*/
+
+ulong Query_cache::get_minimal_size_limit()
+{
+  ulong approx_additional_data_size= (sizeof(Query_cache) +
+                                      sizeof(gptr)*(def_query_hash_size+
+                                      def_table_hash_size));
+
+  ulong data_size= (min_allocation_unit << QUERY_CACHE_MEM_BIN_STEP_PWR2 <<
+                    QUERY_CACHE_MEM_BIN_FIRST_STEP_PWR2) +
+                    ALIGN_SIZE(1) - 1 +
+                    (1 << QUERY_CACHE_MEM_BIN_STEP_PWR2) - 1 +
+                    (1 << QUERY_CACHE_MEM_BIN_FIRST_STEP_PWR2) - 1;
+
+  return(data_size + approx_additional_data_size);
+}
+
 ulong Query_cache::init_cache()
 {
   uint mem_bin_count, num, step;

sql/sql_cache.h

@@ -245,6 +245,8 @@ private:
 
   void free_query_internal(Query_cache_block *point);
 
+  ulong get_minimal_size_limit();
+
 protected:
   /*
     The following mutex is locked when searching or changing global