Optimizer trace: print cost and #rows of the join prefix

The names rows_for_plan and cost_for_plan follow MySQL
Also added post-join-operation selectivity cost

mysql-test/main/opt_trace.test

@@ -508,4 +508,31 @@ SET optimizer_trace = 'enabled=on';
 DELETE FROM t1 WHERE f = 'x';
 DROP TABLE t1;
 
+--echo #
+--echo # Print cost_for_plan and rows_for_plan for join prefix
+--echo #
+create table t0(a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table one_k (a int, b int, key(b));
+insert into one_k select A.a + B.a*10 + C.a*100, A.a + B.a*10 + C.a*100 from t0 A, t0 B, t0 C;
+
+analyze table t0, one_k persistent for all;
+
+set @tmp_jcl=@@join_cache_level;
+set join_cache_level=0;
+set optimizer_trace=1;
+
+--echo # Check cost/row numbers when multiple tables are joined
+--echo #  (cost_for_plan is the same as best_access_path.cost for single-table SELECTs
+--echo #   but for joins using condition selectivity it is not as trivial. So,
+--echo #   now we are printing it)
+explain select * from t0 A, one_k B where A.a<5 and B.a<800;
+select JSON_DETAILED(JSON_EXTRACT(trace, '$**.considered_execution_plans')) from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
+
+set join_cache_level=@tmp_jcl;
+
+--echo # This shows post-join selectivity
+explain select * from t0 A, one_k B where A.a=B.b and B.a<800;
+select JSON_DETAILED(JSON_EXTRACT(trace, '$**.considered_execution_plans')) from INFORMATION_SCHEMA.OPTIMIZER_TRACE;
+
 set optimizer_trace='enabled=off';

mysql-test/main/opt_trace_index_merge.result

@@ -213,7 +213,9 @@ explain select * from t1 where a=1 or b=1	{
                       "chosen": true
                     }
                   ]
-                }
+                },
+                "rows_for_plan": 2,
+                "cost_for_plan": 4.5484
               }
             ]
           },

mysql-test/main/opt_trace_index_merge_innodb.result

@@ -209,7 +209,9 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                       "cause": "cost"
                     }
                   ]
-                }
+                },
+                "rows_for_plan": 1,
+                "cost_for_plan": 2.2
               }
             ]
           },

mysql-test/main/opt_trace_security.result

@@ -99,7 +99,9 @@ select * from db1.t1	{
                       "chosen": true
                     }
                   ]
-                }
+                },
+                "rows_for_plan": 3,
+                "cost_for_plan": 2.6051
               }
             ]
           },
@@ -210,7 +212,9 @@ select * from db1.v1	{
                       "chosen": true
                     }
                   ]
-                }
+                },
+                "rows_for_plan": 3,
+                "cost_for_plan": 2.6051
               }
             ]
           },

sql/sql_select.cc

@@ -9383,6 +9383,11 @@ best_extension_by_limited_search(JOIN      *join,
                                           current_record_count /
                                           (double) TIME_FOR_COMPARE));
 
+      if (unlikely(thd->trace_started()))
+      {
+        trace_one_table.add("rows_for_plan", current_record_count);
+        trace_one_table.add("cost_for_plan", current_read_time);
+      }
       advance_sj_state(join, remaining_tables, idx, &current_record_count,
                        &current_read_time, &loose_scan_pos);
 
@@ -9441,6 +9446,10 @@ best_extension_by_limited_search(JOIN      *join,
 				                          remaining_tables &
                                                           ~real_table_bit);
       join->positions[idx].cond_selectivity= pushdown_cond_selectivity;
+
+      if (unlikely(thd->trace_started()) && pushdown_cond_selectivity < 1.0)
+        trace_one_table.add("selectivity", pushdown_cond_selectivity);
+
       double partial_join_cardinality= current_record_count *
                                         pushdown_cond_selectivity;
       if ( (search_depth > 1) && (remaining_tables & ~real_table_bit) & allowed_tables )