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MDEV-21580: Allow packed sort keys in sort buffer

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This task deals with packing the sort key inside the sort buffer, which  would
lead to efficient usage of the memory allocated for the sort buffer.

The changes brought by this feature are
  1) Sort buffers would have sort keys of variable length
  2) The format for sort keys inside the sort buffer would look like
     |<sort_length><null_byte><key_part1><null_byte><key_part2>.......|
      sort_length is the extra bytes that are required to store the variable
      length of a sort key.
  3) When packing of sort key is done we store the ORIGINAL VALUES inside
     the sort buffer and not the STRXFRM form (mem-comparable sort keys).
  4) Special comparison function packed_keys_comparison() is introduced
     to compare 2 sort keys.

This patch also contains contributions from Sergei Petrunia.
This commit is contained in:
Varun Gupta
2020-03-10 04:56:38 +05:30
parent 561b5ce364
commit b753ac066b
23 changed files with 2075 additions and 299 deletions
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48
sql/sql_class.h
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@ -6277,8 +6277,52 @@ public:
/* Structs used when sorting */
struct SORT_FIELD_ATTR
{
uint length; /* Length of sort field */
uint suffix_length; /* Length suffix (0-4) */
/*
If using mem-comparable fixed-size keys:
length of the mem-comparable image of the field, in bytes.
If using packed keys: still the same? Not clear what is the use of it.
*/
uint length;
/*
For most datatypes, this is 0.
The exception are the VARBINARY columns.
For those columns, the comparison actually compares
(value_prefix(N), suffix=length(value))
Here value_prefix is either the whole value or its prefix if it was too
long, and the suffix is the length of the original value.
(this way, for values X and Y: if X=prefix(Y) then X compares as less
than Y
*/
uint suffix_length;
/*
If using packed keys, number of bytes that are used to store the length
of the packed key.
*/
uint length_bytes;
/* Max. length of the original value, in bytes */
uint original_length;
enum Type { FIXED_SIZE, VARIABLE_SIZE } type;
/*
TRUE : if the item or field is NULLABLE
FALSE : otherwise
*/
bool maybe_null;
CHARSET_INFO *cs;
uint pack_sort_string(uchar *to, const LEX_CSTRING &str,
CHARSET_INFO *cs) const;
int compare_packed_fixed_size_vals(uchar *a, size_t *a_len,
uchar *b, size_t *b_len);
int compare_packed_varstrings(uchar *a, size_t *a_len,
uchar *b, size_t *b_len);
bool check_if_packing_possible(THD *thd) const;
bool is_variable_sized() { return type == VARIABLE_SIZE; }
};