MDEV-16849 Extending indexed VARCHAR column should be instantaneous

Analysis:
========
Increasing the length of the indexed varchar column is not an instant operation for
innodb.

Fix:
===
- Introduce the new handler flag 'Alter_inplace_info::ALTER_COLUMN_INDEX_LENGTH' to
indicate the index length differs due to change of column length changes.

- InnoDB makes the ALTER_COLUMN_INDEX_LENGTH flag as instant operation.

This is a port of Mysql fix.

    commit 913071c0b16cc03e703308250d795bc381627e37
    Author: Nisha Gopalakrishnan <nisha.gopalakrishnan@oracle.com>
    Date:   Wed May 30 14:54:46 2018 +0530

        BUG#26848813: INDEXED COLUMN CAN'T BE CHANGED FROM VARCHAR(15)
                      TO VARCHAR(40) INSTANTANEOUSLY

1 files changed, 27 insertions, 8 deletions

diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index 1b426f80a88..19100aafd51 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -6303,7 +6303,7 @@ static bool fill_alter_inplace_info(THD *thd,
                                     bool varchar,
                                     Alter_inplace_info *ha_alter_info)
 {
-  Field **f_ptr, *field;
+  Field **f_ptr, *field, *old_field;
   List_iterator_fast<Create_field> new_field_it;
   Create_field *new_field;
   KEY_PART_INFO *key_part, *new_part;
@@ -6629,6 +6629,7 @@ static bool fill_alter_inplace_info(THD *thd,
     Go through keys and check if the original ones are compatible
     with new table.
   */
+  uint old_field_len= 0;
   KEY *table_key;
   KEY *table_key_end= table->key_info + table->s->keys;
   KEY *new_key;
@@ -6693,17 +6694,35 @@ static bool fill_alter_inplace_info(THD *thd,
          key_part < end;
          key_part++, new_part++)
     {
+      new_field= get_field_by_index(alter_info, new_part->fieldnr);
+      old_field= table->field[key_part->fieldnr - 1];
       /*
+        If there is a change in index length due to column expansion
+        like varchar(X) changed to varchar(X + N) and has a compatible
+        packed data representation, we mark it for fast/INPLACE change
+        in index definition. InnoDB supports INPLACE for this cases
+
         Key definition has changed if we are using a different field or
-        if the used key part length is different. It makes sense to
-        check lengths first as in case when fields differ it is likely
-        that lengths differ too and checking fields is more expensive
-        in general case.
+        if the user key part length is different.
       */
-      if (key_part->length != new_part->length)
-        goto index_changed;
+      old_field_len= old_field->pack_length();
 
-      new_field= get_field_by_index(alter_info, new_part->fieldnr);
+      if (old_field->type() == MYSQL_TYPE_VARCHAR)
+      {
+        old_field_len= (old_field->pack_length()
+                        - ((Field_varstring*) old_field)->length_bytes);
+      }
+
+      if (key_part->length == old_field_len &&
+          key_part->length < new_part->length &&
+	  (key_part->field->is_equal((Create_field*) new_field)
+           == IS_EQUAL_PACK_LENGTH))
+      {
+        ha_alter_info->handler_flags |=
+           Alter_inplace_info::ALTER_COLUMN_INDEX_LENGTH;
+      }
+      else if (key_part->length != new_part->length)
+        goto index_changed;
 
       /*
         For prefix keys KEY_PART_INFO::field points to cloned Field