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, 9 insertions, 0 deletions

diff --git a/sql/set_var.h b/sql/set_var.h
index 6097b28e76f..8a82e07fd6b 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -275,6 +275,10 @@ public:
   virtual int update(THD *thd)=0;                  /* To set the value */
   virtual int light_check(THD *thd) { return check(thd); }   /* for PS */
   virtual bool is_system() { return FALSE; }
+  /**
+    @returns whether this variable is @@@@optimizer_trace.
+  */
+  virtual bool is_var_optimizer_trace() const { return false; }
 };
 
 
@@ -306,6 +310,11 @@ public:
   int check(THD *thd);
   int update(THD *thd);
   int light_check(THD *thd);
+  virtual bool is_var_optimizer_trace() const
+  {
+    extern sys_var *Sys_optimizer_trace_ptr;
+    return var == Sys_optimizer_trace_ptr;
+  }
 };