MDEV-6111 Optimizer Trace

This task involves the implementation for the optimizer trace.

This feature produces a trace for any SELECT/UPDATE/DELETE/,
which contains information about decisions taken by the optimizer during
the optimization phase (choice of table access method, various costs,
transformations, etc). This feature would help to tell why some decisions were
taken by the optimizer and why some were rejected.

Trace is session-local, controlled by the @@optimizer_trace variable.
To enable optimizer trace we need to write:
   set @@optimizer_trace variable= 'enabled=on';

To display the trace one can run:
   SELECT trace FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;

This task also involves:
    MDEV-18489: Limit the memory used by the optimizer trace
    introduces a switch optimizer_trace_max_mem_size which limits
    the memory used by the optimizer trace. This was implemented by
    Sergei Petrunia.

1 files changed, 24 insertions, 3 deletions

diff --git a/sql/sql_test.cc b/sql/sql_test.cc
index 3d43c35177d..f247fb10f89 100644
--- a/sql/sql_test.cc
+++ b/sql/sql_test.cc
@@ -24,6 +24,7 @@
 #include "sql_show.h" // calc_sum_of_all_status
 #include "sql_select.h"
 #include "keycaches.h"
+#include "my_json_writer.h"
 #include <hash.h>
 #include <thr_alarm.h>
 #if defined(HAVE_MALLINFO) && defined(HAVE_MALLOC_H)
@@ -36,6 +37,8 @@
 #include "events.h"
 #endif
 
+#define FT_KEYPART   (MAX_FIELDS+10)
+
 static const char *lock_descriptions[] =
 {
   /* TL_UNLOCK                  */  "No lock",
@@ -225,8 +228,6 @@ TEST_join(JOIN *join)
 }
 
 
-#define FT_KEYPART   (MAX_FIELDS+10)
-
 static void print_keyuse(KEYUSE *keyuse)
 {
   char buff[256];
@@ -263,7 +264,6 @@ void print_keyuse_array(DYNAMIC_ARRAY *keyuse_array)
   DBUG_UNLOCK_FILE;
 }
 
-
 /* 
   Print the current state during query optimization.
 
@@ -655,3 +655,24 @@ Memory allocated by threads:             %s\n",
   puts("");
   fflush(stdout);
 }
+
+void print_keyuse_array_for_trace(THD *thd, DYNAMIC_ARRAY *keyuse_array)
+{
+  Json_writer_object wrapper(thd);
+  Json_writer_array trace_key_uses(thd, "ref_optimizer_key_uses");
+  for(uint i=0; i < keyuse_array->elements; i++)
+  {
+    KEYUSE *keyuse= (KEYUSE*)dynamic_array_ptr(keyuse_array, i);
+    Json_writer_object keyuse_elem(thd);
+    keyuse_elem.add_table_name(keyuse->table->reginfo.join_tab);
+    keyuse_elem.add("field", (keyuse->keypart == FT_KEYPART) ? "<fulltext>"
+                                      : (keyuse->is_for_hash_join()
+                                        ? keyuse->table->field[keyuse->keypart]
+                                                       ->field_name.str
+                                        : keyuse->table->key_info[keyuse->key]
+                                          .key_part[keyuse->keypart]
+                                          .field->field_name.str));
+    keyuse_elem.add("equals",keyuse->val);
+    keyuse_elem.add("null_rejecting",keyuse->null_rejecting);
+  }
+}