MDEV-16934 Query with very large IN clause lists runs slowly

This patch introduces support for the system variable eq_range_index_dive_limit
that existed in MySQL starting from 5.6. The variable sets a limit for
index dives into equality ranges. Index dives are performed by optimizer
to estimate the number of rows in range scans. Index dives usually provide
good estimate but they are pretty expensive. To estimate the number of rows
in equality ranges statistical data on indexes can be employed. Its usage gives
not so good estimates but it's cheap. So if the number of equality dives
required by an index scan exceeds the set limit no dives for equality
ranges are performed by the optimizer for this index.

As the new system variable is introduced in a stable version the default
value for it is set to a special value meaning there is no limit for the number
of index dives performed by the optimizer.

The patch partially uses the MySQL code for WL 5957
'Statistics-based Range optimization for many ranges'.

1 files changed, 3 insertions, 1 deletions

diff --git a/sql/opt_range.h b/sql/opt_range.h
index c1f7079ce7f..6698c987e78 100644
--- a/sql/opt_range.h
+++ b/sql/opt_range.h
@@ -242,7 +242,7 @@ public:
     Number of children of this element in the RB-tree, plus 1 for this
     element itself.
   */
-  uint16 elements;
+  uint32 elements;
   /*
     Valid only for elements which are RB-tree roots: Number of times this
     RB-tree is referred to (it is referred by SEL_ARG::next_key_part or by
@@ -1664,6 +1664,8 @@ SQL_SELECT *make_select(TABLE *head, table_map const_tables,
 
 bool calculate_cond_selectivity_for_table(THD *thd, TABLE *table, Item **cond);
 
+bool eq_ranges_exceeds_limit(RANGE_SEQ_IF *seq, void *seq_init_param);
+
 #ifdef WITH_PARTITION_STORAGE_ENGINE
 bool prune_partitions(THD *thd, TABLE *table, Item *pprune_cond);
 #endif