MDEV-22360: Sufficient conditions for accurate calculation of join cardinality10.6-mdev22360

The aim of this task is to check if the estimate of join cardinality are accurate or not.
The implementation to check if we have the accurate estimate of the join cardinality is a
simple one, we have to walk over the WHERE clause.

The approach can be broken into 2 cases:

Case 1: WHERE clause is an AND conjunct
	For an AND item at the top level, we need to walk over all the top level conjuncts and call walk
	individually on them. This is done in such a way because for an AND conjunct at the top
	level we may have accurate selectivity, even if the predicate belongs to a different column.
	Eg: t1.a > 10 and t2.a < 5.

	For this AND item we will have accurate selectivities.
	For AND conjuncts (not at the top level), the entire conjunct needs to be resolved to one column.
	Eg: t1.a = t2.a AND ( (t1.a > 5 AND t2.a < 10) OR t1.a <= 0)

Case 2:
	2a) OR item
		For an OR item at the top level, we need to make sure that all the columns inside the OR
		conjunct need to belong to one column directly or indirectly.
		This needs to happen for an OR conjunct even if it is not at the
		top level.
		Eg: (t1.a > 5 or t1.a < 0);

	2b) Single predicate at the top level
	    Eg:
	      t1.a= t2.a [ For this case we need to make sure we know number of distinct values for t1.a and t2.a ]
	      t1.a > 5 [ sargable predicate, get the estimate from the range optimizer ]

We need to make sure that for the predicates in the WHERE clause we have estimates either
from the first component of the index or from the EITS.
The implementation of these is covered with the callback
function passed to walk function.

1 files changed, 40 insertions, 0 deletions

diff --git a/sql/item.h b/sql/item.h
index fb480b4c578..8a6b16ad82d 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -446,6 +446,26 @@ typedef struct replace_equal_field_arg
   struct st_join_table *context_tab;
 } REPLACE_EQUAL_FIELD_ARG;
 
+
+/*
+  Structure storing information for a field on which the entire predicate is
+  dependent on (directly or indirectly via equalities)
+*/
+typedef struct same_field
+{
+  /*
+    field item for the first encountered column while traversing
+    over the conditional predicate
+  */
+  Item *item;
+  /*
+    Set to true if the statistics for the field are available
+    directly (via keys or stat tables) or indirectly (via equalities)
+  */
+  bool is_stats_available;
+}SAME_FIELD;
+
+
 class Settable_routine_parameter
 {
 public:
@@ -1968,7 +1988,25 @@ public:
   virtual bool count_sargable_conds(void *arg) { return 0; }
   virtual bool limit_index_condition_pushdown_processor(void *arg) { return 0; }
   virtual bool exists2in_processor(void *arg) { return 0; }
+
   virtual bool find_selective_predicates_list_processor(void *arg) { return 0; }
+
+  bool with_accurate_selectivity_estimation();
+
+  /*
+    @brief
+      Check if selectivity of a predicate is available via indexes or EITS
+
+    @param
+      arg      Structure storing information whether the AND/OR conjunct
+               can be resolved via a single column.
+
+    @retval
+      FALSE  :  SUCCESS
+      TRUE   :  OTHERWISE
+  */
+  virtual bool predicate_selectivity_checker(void *arg) { return FALSE; }
+  virtual bool dep_on_one_column(void *arg) { return true; }
   bool cleanup_is_expensive_cache_processor(void *arg)
   {
     is_expensive_cache= (int8)(-1);
@@ -3576,6 +3614,7 @@ public:
     return field->table->pos_in_table_list->outer_join;
   }
   bool check_index_dependence(void *arg);
+  bool dep_on_one_column(void *arg);
   friend class Item_default_value;
   friend class Item_insert_value;
   friend class st_select_lex_unit;
@@ -5946,6 +5985,7 @@ public:
   Item *field_transformer_for_having_pushdown(THD *thd, uchar *arg)
   { return this; }
   Item *remove_item_direct_ref() { return this; }
+  bool dep_on_one_column(void *arg);
 };