MDEV-34993, part2: backport optimizer_adjust_secondary_key_costs

...and make the fix for MDEV-34993 switchable. It is enabled by default
and controlled with @optimizer_adjust_secondary_key_costs=fix_card_multiplier

mysql-test/main/mysqld--help.result

@@ -682,6 +682,12 @@ The following specify which files/extra groups are read (specified before remain
  max_connections*5 or max_connections + table_cache*2
  (whichever is larger) number of file descriptors
  (Automatically configured unless set explicitly)
+ --optimizer-adjust-secondary-key-costs=name 
+ A bit field with the following values:
+ fix_card_multiplier = Fix the computation in
+ selectivity_for_indexes. selectivity_multiplier. This
+ variable will be deleted in MariaDB 11.0 as it is not
+ needed with the new 11.0 optimizer.
  --optimizer-max-sel-arg-weight=# 
  The maximum weight of the SEL_ARG graph. Set to 0 for no
  limit
@@ -1642,6 +1648,7 @@ old-alter-table DEFAULT
 old-mode 
 old-passwords FALSE
 old-style-user-limits FALSE
+optimizer-adjust-secondary-key-costs fix_card_multiplier
 optimizer-max-sel-arg-weight 32000
 optimizer-prune-level 1
 optimizer-search-depth 62

mysql-test/main/selectivity_innodb_notembedded.result

@@ -280,7 +280,61 @@ JS
         }
     }
 ]
+# Disable the fix and try the same:
+set @@optimizer_adjust_secondary_key_costs='';
+explain select * from t1 
+where
+pk in (1,2,3,4,5) and
+key1 <= 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY,key1	PRIMARY	4	NULL	5	Using where
 drop table t1;
+# Shows a high multiplier, without a "note":
+select 
+json_detailed(json_extract(trace,'$**.selectivity_for_indexes')) as JS
+from 
+information_schema.optimizer_trace;
+JS
+[
+    [
+        {
+            "index_name": "PRIMARY",
+            "selectivity_from_index": 0.005
+        },
+        {
+            "index_name": "key1",
+            "selectivity_from_index": 0.399,
+            "selectivity_multiplier": 90.9091
+        }
+    ]
+]
+# Includes 1.79...e308 as cost:
+select 
+json_detailed(json_extract(trace,'$**.best_access_path')) as JS
+from 
+information_schema.optimizer_trace;
+JS
+[
+    {
+        "considered_access_paths": 
+        [
+            {
+                "access_type": "range",
+                "resulting_rows": 181.3636545,
+                "cost": 1.79769e308,
+                "chosen": true
+            }
+        ],
+        "chosen_access_method": 
+        {
+            "type": "range",
+            "records": 181.3636545,
+            "cost": 1.79769e308,
+            "uses_join_buffering": false
+        }
+    }
+]
+set optimizer_adjust_secondary_key_costs=default;
 #
 # Clean up
 #

mysql-test/main/selectivity_notembedded.result

@@ -275,7 +275,61 @@ JS
         }
     }
 ]
+# Disable the fix and try the same:
+set @@optimizer_adjust_secondary_key_costs='';
+explain select * from t1 
+where
+pk in (1,2,3,4,5) and
+key1 <= 4;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY,key1	PRIMARY	4	NULL	5	Using index condition; Using where
 drop table t1;
+# Shows a high multiplier, without a "note":
+select 
+json_detailed(json_extract(trace,'$**.selectivity_for_indexes')) as JS
+from 
+information_schema.optimizer_trace;
+JS
+[
+    [
+        {
+            "index_name": "PRIMARY",
+            "selectivity_from_index": 0.005
+        },
+        {
+            "index_name": "key1",
+            "selectivity_from_index": 0.391,
+            "selectivity_multiplier": 90.9091
+        }
+    ]
+]
+# Includes 1.79...e308 as cost:
+select 
+json_detailed(json_extract(trace,'$**.best_access_path')) as JS
+from 
+information_schema.optimizer_trace;
+JS
+[
+    {
+        "considered_access_paths": 
+        [
+            {
+                "access_type": "range",
+                "resulting_rows": 177.7272905,
+                "cost": 1.79769e308,
+                "chosen": true
+            }
+        ],
+        "chosen_access_method": 
+        {
+            "type": "range",
+            "records": 177.7272905,
+            "cost": 1.79769e308,
+            "uses_join_buffering": false
+        }
+    }
+]
+set optimizer_adjust_secondary_key_costs=default;
 #
 # Clean up
 #

mysql-test/main/selectivity_notembedded.test

@@ -257,7 +257,25 @@ select
 from 
   information_schema.optimizer_trace;
 
+--echo # Disable the fix and try the same:
+set @@optimizer_adjust_secondary_key_costs='';
+explain select * from t1 
+where
+  pk in (1,2,3,4,5) and
+  key1 <= 4;
 drop table t1;
+--echo # Shows a high multiplier, without a "note":
+select 
+  json_detailed(json_extract(trace,'$**.selectivity_for_indexes')) as JS
+from 
+  information_schema.optimizer_trace;
+
+--echo # Includes 1.79...e308 as cost:
+select 
+  json_detailed(json_extract(trace,'$**.best_access_path')) as JS
+from 
+  information_schema.optimizer_trace;
+set optimizer_adjust_secondary_key_costs=default;
 
 --echo #
 --echo # Clean up

mysql-test/suite/sys_vars/r/sysvars_server_notembedded.result

@@ -2402,6 +2402,16 @@ NUMERIC_BLOCK_SIZE	1
 ENUM_VALUE_LIST	NULL
 READ_ONLY	YES
 COMMAND_LINE_ARGUMENT	REQUIRED
+VARIABLE_NAME	OPTIMIZER_ADJUST_SECONDARY_KEY_COSTS
+VARIABLE_SCOPE	SESSION
+VARIABLE_TYPE	SET
+VARIABLE_COMMENT	A bit field with the following values: fix_card_multiplier = Fix the computation in selectivity_for_indexes. selectivity_multiplier. This variable will be deleted in MariaDB 11.0 as it is not needed with the new 11.0 optimizer.
+NUMERIC_MIN_VALUE	NULL
+NUMERIC_MAX_VALUE	NULL
+NUMERIC_BLOCK_SIZE	NULL
+ENUM_VALUE_LIST	fix_card_multiplier
+READ_ONLY	NO
+COMMAND_LINE_ARGUMENT	REQUIRED
 VARIABLE_NAME	OPTIMIZER_MAX_SEL_ARG_WEIGHT
 VARIABLE_SCOPE	SESSION
 VARIABLE_TYPE	BIGINT UNSIGNED

sql/opt_range.cc

@@ -3521,7 +3521,9 @@ bool calculate_cond_selectivity_for_table(THD *thd, TABLE *table, Item **cond)
               multiplied by quick_cond_selectivity above, so we will only
               multiply it with selectivity_mult).
             */
-            if (selectivity_mult > 1.0 / quick_cond_selectivity)
+            if ((thd->variables.optimizer_adjust_secondary_key_costs &
+                 OPTIMIZER_ADJ_FIX_CARD_MULT) &&
+                selectivity_mult > 1.0 / quick_cond_selectivity)
             {
               selectivity_for_index.add("note", "multiplier too high, clipping");
               selectivity_mult= 1.0/quick_cond_selectivity;

sql/sql_class.h

@@ -672,6 +672,11 @@ typedef struct system_variables
   ulonglong sortbuff_size;
   ulonglong default_regex_flags;
   ulonglong max_mem_used;
+  /*
+    A bitmap of OPTIMIZER_ADJ_* flags (defined in sql_priv.h).
+    See sys_vars.cc:adjust_secondary_key_cost for symbolic names.
+  */
+  ulonglong optimizer_adjust_secondary_key_costs;
 
   /**
      Place holders to store Multi-source variables in sys_var.cc during

sql/sql_priv.h

@@ -266,6 +266,11 @@
                                     OPTIMIZER_SWITCH_COND_PUSHDOWN_FROM_HAVING | \
                                     OPTIMIZER_SWITCH_OPTIMIZE_JOIN_BUFFER_SIZE)
 
+/*
+  See adjust_secondary_key_cost in sys_vars.cc for symbolic names.
+*/
+#define OPTIMIZER_ADJ_FIX_CARD_MULT (1)
+
 /*
   Replication uses 8 bytes to store SQL_MODE in the binary log. The day you
   use strictly more than 64 bits by adding one more define above, you should

sql/sys_vars.cc

@@ -2777,6 +2777,25 @@ static Sys_var_ulong Sys_optimizer_trace_max_mem_size(
     SESSION_VAR(optimizer_trace_max_mem_size), CMD_LINE(REQUIRED_ARG),
     VALID_RANGE(0, ULONG_MAX), DEFAULT(1024 * 1024), BLOCK_SIZE(1));
 
+/*
+  Symbolic names for OPTIMIZER_ADJ_* flags in sql_priv.h
+*/
+static const char *adjust_secondary_key_cost[]=
+{
+  "fix_card_multiplier", 0
+};
+
+static Sys_var_set Sys_optimizer_adjust_secondary_key_costs(
+    "optimizer_adjust_secondary_key_costs",
+    "A bit field with the following values: "
+    "fix_card_multiplier = Fix the computation in selectivity_for_indexes."
+    " selectivity_multiplier. "
+    "This variable will be deleted in MariaDB 11.0 as it is not needed with the "
+    "new 11.0 optimizer.",
+    SESSION_VAR(optimizer_adjust_secondary_key_costs), CMD_LINE(REQUIRED_ARG),
+    adjust_secondary_key_cost, DEFAULT(OPTIMIZER_ADJ_FIX_CARD_MULT));
+
+
 static Sys_var_charptr_fscs Sys_pid_file(
        "pid_file", "Pid file used by safe_mysqld",
        READ_ONLY GLOBAL_VAR(pidfile_name_ptr), CMD_LINE(REQUIRED_ARG),