typedef struct st_key {

  uint    key_length;            /* Tot length of key */

  ulong flags;                          /* dupp key and pack flags */

  uint    key_parts;            /* How many key_parts */ //一共有多少个column

  uint  extra_length;

  uint    usable_key_parts;        /* Should normally be = key_parts */

  uint  block_size;

  enum  ha_key_alg algorithm;

  /*

    Note that parser is used when the table is opened for use, and

    parser_name is used when the table is being created.

  */

  union

  {

    plugin_ref parser;                  /* Fulltext [pre]parser */

    LEX_STRING *parser_name;            /* Fulltext [pre]parser name */

  };

  KEY_PART_INFO *key_part; //key中具体的column
char    *name;                /* Name of key */

  /*

    Array of AVG(#records with the same field value) for 1st ... Nth key part.

    0 means 'not known'.

    For temporary heap tables this member is NULL.

  */

  ulong *rec_per_key;

  union {

    int  bdb_return_if_eq;

  } handler;

  TABLE *table;

  LEX_STRING comment;

} KEY;

typedef struct st_key_part_info {    /* Info about a key part */

  Field *field;

  uint    offset;                /* offset in record (from 0) */

  uint    null_offset;            /* Offset to null_bit in record */

  uint16 length;                        /* Length of keypart value in bytes */

  /*

    Number of bytes required to store the keypart value. This may be

    different from the "length" field as it also counts

     - possible NULL-flag byte (see HA_KEY_NULL_LENGTH)

     - possible HA_KEY_BLOB_LENGTH bytes needed to store actual value length.

  */

  uint16 store_length;

  uint16 key_type;

  uint16 fieldnr;            /* Fieldnum in UNIREG */

  uint16 key_part_flag;            /* 0 or HA_REVERSE_SORT */

  uint8 type;

  uint8 null_bit;            /* Position to null_bit */

} KEY_PART_INFO ;

typedef struct st_keyfile_info {    /* used with ha_info() */

  uchar ref[MAX_REFLENGTH];        /* Pointer to current row */

  uchar dupp_ref[MAX_REFLENGTH];    /* Pointer to dupp row */

  uint ref_length;            /* Length of ref (1-8) */

  uint block_size;            /* index block size */

  File filenr;                /* (uniq) filenr for table */

  ha_rows records;            /* Records i datafilen */

  ha_rows deleted;            /* Deleted records */

  ulonglong data_file_length;        /* Length off data file */

  ulonglong max_data_file_length;    /* Length off data file */

  ulonglong index_file_length;

  ulonglong max_index_file_length;

  ulonglong delete_length;        /* Free bytes */

  ulonglong auto_increment_value;

  int errkey,sortkey;            /* Last errorkey and sorted by */

  time_t create_time;            /* When table was created */

  time_t check_time;

  time_t update_time;

  ulong mean_rec_length;        /* physical reclength */

} KEYFILE_INFO;