MDEV-21265: IN predicate conversion to IN subquery should be allowed for a broader set of datatype comparisonbb-10.3-21265

Allow materialization strategy when collations on the
inner and outer sides of an IN subquery are the same and the
character set of the inner side is a proper subset of the character
set on the outer side.
This allows conversion from utf8mb3 to utf8mb4
as the former is a subset of the later.
This is only allowed when IN predicate is converted to an IN subquery

Backported part of the patch (d6a00d9b18f) of MDEV-17905.

1 files changed, 18 insertions, 0 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index 11c3f0cd573..4f9a68acbb4 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -131,6 +131,24 @@ uint convert_to_printable(char *to, size_t to_len,
                           const char *from, size_t from_len,
                           CHARSET_INFO *from_cs, size_t nbytes= 0);
 
+class Charset
+{
+  CHARSET_INFO *m_charset;
+public:
+  Charset() :m_charset(&my_charset_bin) { }
+  Charset(CHARSET_INFO *cs) :m_charset(cs) { }
+
+  CHARSET_INFO *charset() const { return m_charset; }
+  /*
+    Collation name without the character set name.
+    For example, in case of "latin1_swedish_ci",
+    this method returns "_swedish_ci".
+  */
+  LEX_CSTRING collation_specific_name() const;
+  bool encoding_allows_reinterpret_as(CHARSET_INFO *cs) const;
+  bool eq_collation_specific_names(CHARSET_INFO *cs) const;
+};
+
 class String : public Sql_alloc
 {
   char *Ptr;