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, 92 insertions, 0 deletions

diff --git a/sql/opt_trace_context.h b/sql/opt_trace_context.h
new file mode 100644
index 00000000000..87317f67e22
--- /dev/null
+++ b/sql/opt_trace_context.h
@@ -0,0 +1,92 @@
+#ifndef OPT_TRACE_CONTEXT_INCLUDED
+#define OPT_TRACE_CONTEXT_INCLUDED
+
+#include "sql_array.h"
+
+class Opt_trace_stmt;
+
+class Opt_trace_context
+{
+public:
+   Opt_trace_context();
+  ~Opt_trace_context();
+
+  void start(THD *thd, TABLE_LIST *tbl,
+             enum enum_sql_command sql_command,
+             const char *query,
+             size_t query_length,
+             const CHARSET_INFO *query_charset,
+             ulong max_mem_size_arg);
+  void end();
+  void set_query(const char *query, size_t length, const CHARSET_INFO *charset);
+  void flush_optimizer_trace();
+  void set_allowed_mem_size(size_t mem_size);
+  size_t remaining_mem_size();
+
+private:
+  Opt_trace_stmt* top_trace()
+  {
+    return *(traces->front());
+  }
+
+public:
+
+  /*
+    This returns the top trace from the list of traces. This function
+    is used when we want to see the contents of the INFORMATION_SCHEMA.OPTIMIZER_TRACE
+  table.
+  */
+
+  Opt_trace_stmt* get_top_trace()
+  {
+    if (!traces || !traces->elements())
+      return NULL;
+    return top_trace();
+  }
+
+  /*
+    This returns the current trace, to which we are still writing and has not been finished
+  */
+
+  Json_writer* get_current_json();
+
+  bool empty()
+  {
+    return traces && (static_cast<uint>(traces->elements()) != 0);
+  }
+
+  bool is_started()
+  {
+    return current_trace && is_enabled();
+  }
+
+  bool disable_tracing_if_required();
+
+  bool enable_tracing_if_required();
+
+  bool is_enabled();
+
+  void missing_privilege();
+
+  static const char *flag_names[];
+  enum
+  {
+    FLAG_DEFAULT = 0,
+    FLAG_ENABLED = 1 << 0
+  };
+
+private:
+  /*
+    List of traces (currently it stores only 1 trace)
+  */
+  Dynamic_array<Opt_trace_stmt*> *traces;
+  Opt_trace_stmt *current_trace;
+  /*
+    TRUE: if we allocate memory for list of traces
+    FALSE: otherwise
+  */
+  bool inited;
+  size_t max_mem_size;
+};
+
+#endif /* OPT_TRACE_CONTEXT_INCLUDED */