数据库内核月报 － 2020 / 03

// 返回用于存储字段数量的字节数
uint8_t rec_get_n_fields_length(ulint n_fields) {
  return (n_fields > REC_N_FIELDS_ONE_BYTE_MAX ? 2 : 1);
}

// 设置字段数量
uint8_t rec_set_n_fields(rec_t *rec, ulint n_fields) {
  // 指向记录头信息的前一个字节
  byte *ptr = rec - (REC_N_NEW_EXTRA_BYTES + 1);

  ut_ad(n_fields < REC_MAX_N_FIELDS);
  
  // 如果字段数量小于或等于127
  if (n_fields <= REC_N_FIELDS_ONE_BYTE_MAX) {
    // 在当前位置存储字段数量
    *ptr = static_cast<byte>(n_fields);
    // 存储字段数量的字节数是1
    return (1);
  }
  
  // 如果字段数量大于127，向前移动一个字节
  --ptr;
  // 第一个字节记录低8位数据
  *ptr++ = static_cast<byte>(n_fields & 0xFF);
  // 第二个字节记录高8位数据
  *ptr = static_cast<byte>(n_fields >> 8);
  ut_ad((*ptr & 0x80) == 0);
  *ptr |= REC_N_FIELDS_TWO_BYTES_FLAG;
  
  // 存储字段数量的字节数是2
  return (2);
}

$ hexdump -C -v t1.ibd > t1.txt

00010070  73 75 70 72 65 6d 75 6d  03 0a 02 01 00 00 00 10  |supremum........|
00010080  00 29 80 00 00 01 00 00  00 00 07 d8 9e 00 00 00  |.)..............|
00010090  94 01 10 61 61 62 61 62  20 20 20 20 20 20 20 20  |...aabab        |
000100a0  63 63 63 03 01 06 00 00  18 00 1f 80 00 00 02 00  |ccc.............|
000100b0  00 00 00 07 d9 9f 00 00  00 94 01 10 62 64 64 64  |............bddd|

$ hexdump -C -v t1.ibd > t1.txt

00010070  73 75 70 72 65 6d 75 6d  03 0a 02 01 00 00 00 10  |supremum........|
00010080  00 29 80 00 00 01 00 00  00 00 07 d8 9e 00 00 00  |.)..............|
00010090  94 01 10 61 61 62 61 62  20 20 20 20 20 20 20 20  |...aabab        |
000100a0  63 63 63 03 01 06 00 00  18 00 1f 80 00 00 02 00  |ccc.............|
000100b0  00 00 00 07 d9 9f 00 00  00 94 01 10 62 64 64 64  |............bddd|
000100c0  04 03 01 06 08 80 00 20  ff a6 80 00 00 03 00 00  |....... ........|
000100d0  00 00 07 e7 a0 00 00 00  95 01 10 63 65 65 65 65  |...........ceeee|
000100e0  65 65 65 00 00 00 00 00  00 00 00 00 00 00 00 00  |eee.............|

  |-mysql_execute_command
    |-Sql_cmd_dml::execute
      |-Sql_cmd_dml::execute_inner
        |-JOIN::exec
          |-do_select
            |-sub_select
              |-TableScanIterator::Read
                |-handler::ha_rnd_next
                  |-ha_innobase::rnd_next
                    |-ha_innobase::index_first
                      |-ha_innobase::index_read
                        |-row_search_mvcc
                          |-rec_get_offsets_func
                            |-rec_init_offsets
                              |-rec_init_offsets_comp_ordinary
                                |-rec_init_null_and_len_comp
                                  |-*nulls = rec - (REC_N_NEW_EXTRA_BYTES + 1); // REC_N_NEW_EXTRA_BYTES = 5, 
                                  |-if (!index->has_instant_cols())
                                    |-*n_null = index->n_nullable;
                                  |-else if (rec_get_instant_flag_new(rec) /* Row inserted after first instant ADD COLUMN */
                                    |-non_default_fields = rec_get_n_fields_instant
                                    |-*nulls -= length;
                                    |-*n_null = index->get_n_nullable_before(non_default_fields);
                                  |-else /* Row inserted before first instant ADD COLUMN */
                                    |-*n_null = index->n_instant_nullable;
                                    |-non_default_fields = index->get_instant_fields();
                          |-row_sel_store_mysql_rec
                            |-for (i = 0; i < prebuilt->n_template; i++) 
                              |-row_sel_store_mysql_field // row_sel_store_mysql_field_func
                                |-rec_get_nth_field_instant // 如果是记录中的，则从记录中读取，否则返回其默认值
                                |-row_sel_field_store_in_mysql_format_func

bool rec_convert_dtuple_to_rec_comp(rec_t *rec, const dict_index_t *index,
                                    const dfield_t *fields, ulint n_fields,
                                    const dtuple_t *v_entry, ulint status,
                                    bool temp) {
  const dfield_t *field;
  const dtype_t *type;
  byte *end;
  byte *nulls;
  byte *lens = NULL;
  ulint len;
  ulint i;
  ulint n_node_ptr_field;
  ulint fixed_len;
  ulint null_mask = 1;
  ulint n_null = 0;
  ulint num_v = v_entry ? dtuple_get_n_v_fields(v_entry) : 0;
  bool instant = false;

  ut_ad(temp || dict_table_is_comp(index->table));

  if (n_fields != 0) {
    // 获得nullable字段个数
    n_null = index->has_instant_cols()
                 ? index->get_n_nullable_before(static_cast<uint32_t>(n_fields))
                 : index->n_nullable;
  }

  if (temp) {
    ut_ad(status == REC_STATUS_ORDINARY);
    ut_ad(n_fields <= dict_index_get_n_fields(index));
    n_node_ptr_field = ULINT_UNDEFINED;
    nulls = rec - 1;
    if (dict_table_is_comp(index->table)) {
      /* No need to do adjust fixed_len=0. We only
      need to adjust it for ROW_FORMAT=REDUNDANT. */
      temp = false;
    }
  } else {
    ut_ad(v_entry == NULL);
    ut_ad(num_v == 0);
    // 指向指向记录头信息的前一个字节
    nulls = rec - (REC_N_NEW_EXTRA_BYTES + 1);

    switch (UNIV_EXPECT(status, REC_STATUS_ORDINARY)) {
      case REC_STATUS_ORDINARY:
        ut_ad(n_fields <= dict_index_get_n_fields(index));
        n_node_ptr_field = ULINT_UNDEFINED;
        
        // 如果存在instant column，那么还存在字段个数信息，调用rec_set_n_fields设置
        // 字段数量，并返回存储字节数, 如果字段数量不大于127，存储长度为1字节，否则为2字节
        if (index->has_instant_cols()) {
          uint32_t n_fields_len;
          n_fields_len = rec_set_n_fields(rec, n_fields);
          // nulls指向存储字段数量信息的前一个字节，也就是null标志位最后一个字节开始的位置
          nulls -= n_fields_len;
          instant = true;
        }
        break;
      case REC_STATUS_NODE_PTR:
        ut_ad(n_fields ==
              static_cast<ulint>(
                  dict_index_get_n_unique_in_tree_nonleaf(index) + 1));
        n_node_ptr_field = n_fields - 1;
        n_null = index->n_instant_nullable;
        break;
      case REC_STATUS_INFIMUM:
      case REC_STATUS_SUPREMUM:
        ut_ad(n_fields == 1);
        n_node_ptr_field = ULINT_UNDEFINED;
        break;
      default:
        ut_error;
        return (instant);
    }
  }

  end = rec;

  if (n_fields != 0) {
    // 指向变长字段长度列表最后一个字节开始的位置
    lens = nulls - UT_BITS_IN_BYTES(n_null);
    /* clear the SQL-null flags */
    memset(lens + 1, 0, nulls - lens);
  }

  /* Store the data and the offsets */
  
  // 遍历所有字段
  for (i = 0; i < n_fields; i++) {
    const dict_field_t *ifield;
    dict_col_t *col = NULL;

    field = &fields[i];

    type = dfield_get_type(field);
    len = dfield_get_len(field);

    if (UNIV_UNLIKELY(i == n_node_ptr_field)) {
      ut_ad(dtype_get_prtype(type) & DATA_NOT_NULL);
      ut_ad(len == REC_NODE_PTR_SIZE);
      memcpy(end, dfield_get_data(field), len);
      end += REC_NODE_PTR_SIZE;
      break;
    }
    
    // 如果不是not null类型的字段
    if (!(dtype_get_prtype(type) & DATA_NOT_NULL)) {
      /* nullable field */
      ut_ad(n_null--);
      
      // 如果写满8个，则offset向左移1位，并将null_mask置为1
      if (UNIV_UNLIKELY(!(byte)null_mask)) {
        nulls--;
        null_mask = 1;
      }

      ut_ad(*nulls < null_mask);

      // 如果字段是null
      if (dfield_is_null(field)) {
        // 将null标志位设为1
        *nulls |= null_mask;
        // 向前移1位
        null_mask <<= 1;
        continue;
      }

      null_mask <<= 1;
    }
    /* only nullable fields can be null */
    ut_ad(!dfield_is_null(field));

    ifield = index->get_field(i);
    fixed_len = ifield->fixed_len;
    col = ifield->col;
    if (temp && fixed_len && !col->get_fixed_size(temp)) {
      fixed_len = 0;
    }

    /* If the maximum length of a variable-length field
    is up to 255 bytes, the actual length is always stored
    in one byte. If the maximum length is more than 255
    bytes, the actual length is stored in one byte for
    0..127.  The length will be encoded in two bytes when
    it is 128 or more, or when the field is stored externally. */
    if (fixed_len) {
#ifdef UNIV_DEBUG
      ulint mbminlen = DATA_MBMINLEN(col->mbminmaxlen);
      ulint mbmaxlen = DATA_MBMAXLEN(col->mbminmaxlen);

      ut_ad(len <= fixed_len);
      ut_ad(!mbmaxlen || len >= mbminlen * (fixed_len / mbmaxlen));
      ut_ad(!dfield_is_ext(field));
#endif /* UNIV_DEBUG */
    } else if (dfield_is_ext(field)) {
      ut_ad(DATA_BIG_COL(col));
      ut_ad(len <= REC_ANTELOPE_MAX_INDEX_COL_LEN + BTR_EXTERN_FIELD_REF_SIZE);
      *lens-- = (byte)(len >> 8) | 0xc0;
      *lens-- = (byte)len;
    } else {
      /* DATA_POINT would have a fixed_len */
      ut_ad(dtype_get_mtype(type) != DATA_POINT);
#ifndef UNIV_HOTBACKUP
      ut_ad(len <= dtype_get_len(type) ||
            DATA_LARGE_MTYPE(dtype_get_mtype(type)) ||
            !strcmp(index->name, FTS_INDEX_TABLE_IND_NAME));
#endif /* !UNIV_HOTBACKUP */
      if (len < 128 ||
          !DATA_BIG_LEN_MTYPE(dtype_get_len(type), dtype_get_mtype(type))) {
        *lens-- = (byte)len;
      } else {
        ut_ad(len < 16384);
        // 设置变长字段长度信息
        *lens-- = (byte)(len >> 8) | 0x80;
        *lens-- = (byte)len;
      }
    }
    if (len > 0) memcpy(end, dfield_get_data(field), len);
    end += len;
  }

  if (!num_v) {
    return (instant);
  }

  /* reserve 2 bytes for writing length */
  byte *ptr = end;
  ptr += 2;

  ......
  mach_write_to_2(end, ptr - end);

  return (instant);
}