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YeltsinDB/table_file_v1.md

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Table file structure (v1)

Table file version changelog

v1.0

  • Added previous page offset in page.

v1.x specification

  1. TBL! file signature (4 bytes) could be TBL? if an operation on a table is incompleted
  2. Table file version (2 bytes)
  3. The offset to the first page in a table. (8 bytes)
  4. The offset to the last page in a table. (8 bytes)
  5. The offset to the last available free page (8 bytes)
  6. Pages (64 KiB each)
    1. Page flags (1 byte)
    2. Next page offset (8 bytes) could be 0 if last page
    3. Previous page offset (8 bytes) could be 0 if first page
    4. Row count (2 bytes)
    5. Rows
      1. Row flags (1 byte)
      2. Row data

Table file version specification

In the further time, it's planned to rewrite the file structure, so it's useful to have a version marker.

15-8 7-0
Major version Minor version

That means that the first byte defines major version, and the second one defines minor version.

If tables with different versions are binary-compatible with each other, their major version must be the same.

Page flags specification

7 6 5 4 3 2 1 0
RSV RSV RSV RSV RSV RSV RSV DEL
  • RSV -- reserved for further usage.
  • DEL -- free page flag.

Row flags specification

See "Page flags specification" above.

Free pages

A page can be called free iff all its rows are deleted. If there is a free page, there actions are being done:

  1. Set DEL page flag ((6.1) |= FLAG_DEL)
  2. If a page is not the first one, replace next page offset in the previous page with a value in current page ((previous 6.2) = (6.2))
  3. If a page is not the last one, replace previous page offset in the next page with a value in current page ((previous 6.3) = (6.3))
  4. Put last available free page as the next page ((6.2) = 5)
  5. Set current page offset as the offset to the last available free page ((5) = current_page_offset)

File signature

Since v0.2 a file signature could be TBL?, which signals for incomplete table write operation. If that signature is detected, the state of a table should be reverted to that it was before failed transaction.