MDEV-21829: Use packed sort keys in Unique objectsbb-10.6-mdev21829

The task deals with packing the values stored in the Unique tree for each record.

The changes brought by this feature is:

  1) Unique tree can have dynamic length keys
  2) Format of keys looks like
     <key_length> <packed_value1> <packed_value2> ....... <packed_valueN>

Unique class is currently used in

1) agg_func(DISTINCT col)
   Here most aggregate functions like SUM, AVG accept only fixed size arguments
   so it is not beneficial to use packing for these. Packing is done for
   COUNT and GROUP_CONCAT (or JSON_ARRAYAGG) aggregate function as these are meaningful

2) index-merge stores row-ids
   index merge stores row-ids which are of fixed size, so packing is not required

3) Engine Independent Table statistics
   Packing is done here for variable length data types

This task is an extension to MDEV-21580.

1 files changed, 41 insertions, 0 deletions

diff --git a/sql/field.cc b/sql/field.cc
index 2173670572d..587cbdfb8c3 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -1064,6 +1064,47 @@ Field::make_packed_sort_key_part(uchar *buff,
 }
 
 
+/*
+  @brief
+    Create a packed sort key part
+
+  @param  buff           buffer where values are written
+  @param  sort_field     sort column structure
+
+  @details
+   This function stores the original values for the fixed size columns and
+   does not covert them to mem-comparable images.
+
+  @retval
+    length of the bytes written, does not include the NULL bytes
+*/
+
+uint
+Field::make_packed_key_part(uchar *buff, const SORT_FIELD_ATTR *sort_field)
+{
+  if (maybe_null())
+  {
+    if (is_null())
+    {
+      *buff++= 0;
+      return 0;  // For NULL values don't write any data
+    }
+    *buff++=1;
+  }
+  memcpy(buff, ptr, sort_field->original_length);
+  return sort_field->original_length;
+}
+
+
+uint
+Field_longstr::make_packed_key_part(uchar *buff,
+                                    const SORT_FIELD_ATTR *sort_field)
+{
+  return make_packed_sort_key_part(buff, sort_field);
+}
+
+
+
 uint
 Field_longstr::make_packed_sort_key_part(uchar *buff,
                                          const SORT_FIELD_ATTR *sort_field)