Description:
The flag HA_KEY_HAS_PART_KEY_SEG does not get passed to storage engines in the
key->flags.

The reason is the following. Although properly set in mysql_prepare_create_table, in the
function pack_keys in unireg.cc, only 2 bytes are stored for a key's flags. This causes
HA_KEY_HAS_PART_KEY_SEG to be truncated, because it maps to 0x10000.

How to repeat:
Do the following:
create table foo (a varchar(10), b int, key (a(3)));

Break in mysqld!pack_keys

Notice how keyinfo[0].flags = 0x10040

Break in mysqld!ha_myisam::create

Notice how table_arg->key_info[0].flags = 0x40

Suggested fix:
4 bytes should be stored for a key's flags. KEY->flags is a ulong, so it makes sense to
store more than 2 bytes.

Thank you for the bug report. Verified as described:

-	keyinfo	0x01f4efc0 {key_length=0x00000009 flags=0x00010068 key_parts=0x00000001
...}	st_key *
	key_length	0x00000009	unsigned int
	flags	0x00010068	unsigned long
	key_parts	0x00000001	unsigned int
	extra_length	0x00000000	unsigned int
	usable_key_parts	0x00000000	unsigned int
	block_size	0x00000000	unsigned int
	algorithm	HA_KEY_ALG_UNDEF	ha_key_alg
+	parser	0x00000000	st_plugin_int * *
<cut>

-	table_arg	0x0392d8ac {s=0x0392d458 {table_category=TABLE_CATEGORY_USER
name_hash={key_offset=0x00000000 key_length=0x00000000 blength=0x00000000 ...}
mem_root={free=0x01f2dc50 {next=0x00000000 left=0x000000f8 size=0x000003c8 }
used=0x00000000 {next=??? left=??? size=??? } pre_alloc=0x00000000 {next=??? left=???
size=??? } ...} ...} file=0x01f54a10 {file=0x00000000 {s=??? state=???
save_state={records=??? del=??? empty=??? ...} ...} int_table_flags=0x0000000d14626d91
data_file_name=0xa5a5a5a5 <Bad Ptr> ...} next=0x00000000 {s=??? file=??? next=??? ...}
...}	st_table *
<cut>
-	key_info	0x01f54c40 {key_length=0x0000000c flags=0x00000068 key_parts=0x00000001
...}	st_key *
	key_length	0x0000000c	unsigned int
	flags	0x00000068	unsigned long
	key_parts	0x00000001	unsigned int
	extra_length	0x00000003	unsigned int
	usable_key_parts	0x00000001	unsigned int
	block_size	0x00000000	unsigned int
	algorithm	HA_KEY_ALG_UNDEF	ha_key_alg
+	parser	0x00000000	st_plugin_int * *
<cut>

I discussed this issue with Sergei Golubchik.

It is not a bug, because HA_KEY_HAS_PART_KEY_SEG flag is
internal to the MySQL server by design. It is not supposed
neither to be saved in FRM-files, nor to be passed to storage
engines (it's intended to pass information into internal
static sort_keys(KEY *, KEY *) function)

This flag can actually be calculated -- it's based on key lengths
comparison. So, a storage engine can determine it by itself.

The main motivation for filing this bug is that the fact that the .frm file only uses 2
bytes to store key flags can be a problem. The internal data structure uses 4 bytes. So,
the key flags cannot be extended. I will file a separate bug on this.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/78300

2826 Alexander Nozdrin	2009-07-09
      Add a comment in the source (Bug#40358).