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Access MDB File Format

This file documents the Microsoft Access MDB file format for Jet3 and Jet4 databases.

General Notes
-------------

Access (Jet) does not in general initialize pages to zero before writing them,
so the file will contains a lot of unititialized data. This makes the task of
figuring out the format a bit more difficult than it otherwise would be.

This document will, generally speaking, provide all offsets and constants in
hex format.

Most multibyte pointer and integers are stored in little endian (LSB-MSB) order.
There is an exception in the case of indexes, see the section on index pages for
details.

Terminology
-----------

This section contains a mix of information about data structures used in the MDB
file format along with general database terminology needed to explain these
structures.

Page - A fixed size region within the file on a 2 or 4K boundry. All
data in the file exists inside pages.
Catalog Table - Tables in Access generally starting with "MSys". See the TDEF
(table definition) pages for "System Table" field.
Catalog Entry - A row from the MSysObjects table describing another database
object. The MSysObjects table definition page is always at
page 2 of the database, and a phony tdef structure is
bootstrapped to initially read the database.
Page Split - A process in which a row is added to a page with no space left.
A second page is allocated and rows on the original page are
split between the two pages and then indexes are updated. Pages
can use a variety of algorithms for splitting the rows, the
most popular being a 50/50 split in which rows are divided
evenly between pages.
Overflow Page - Instead of doing a full page split with associated index writes,
a pointer to an "overflow" page can be stored at the original
row's location. Compacting a database would normally rewrite
overflow pages back into regular pages.
Leaf Page - The lowest page on an index tree. In Access, leaf pages are of
a different type than other index pages.
UCS-2 - a two byte unicode encoding used in Jet4 files.
Covered Query - a query that can be satisfied by reading only index pages. For
instance if the query
"SELECT count(*) from Table1 where Column3 = 4" were run and
Column3 was indexed, the query could be satisfied by reading
only indexes. Because of the way Access hashes text columns
in indexes, covered queries on text columns are not possible.

Pages
-----

At it's topmost level MDB files are organized into a series of fixed sized
pages. These are 2K in size for Jet3 (Access 97) and 4K for Jet4 (Access
2000/2002). All data in MDB files exists within pages, of which there are
a number of types.

The first byte of each page idenitifies the page type as follows.

0x00 Database definition page. (Always page 0)
0x01 Data page
0x02 Table definition
0x03 Intermediate Index pages
0x04 Leaf Index pages
0x05 Page Usage Bitmaps (extended page usage)

Database Definition Page
------------------------

Each MDB database has a single definition page located at beginning of the file.
Not a lot is known about this page, and it is one of the least documented page
types. However, it contains things like Jet version, encryption keys, and name
of the creating program.

Offset 0x14 contains the Jet version of this database 0x00 for 3, 0x01 for 4
This is used by the mdb-ver utility to determine the Jet version.

Data Pages
----------

All data rows are stored in type 0x01 pages.

The header of a Jet3 data page looks like this:

+--------------------------------------------------------------------------+
| Jet3 Data Page Definition |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| 0x01 | 1 byte | page_type | 0x01 indicates a data page. |
| 0x01 | 1 byte | unknown | |
| ???? | 2 bytes | free_space | Free space in this page |
| ???? | 4 bytes | tdef_pg | Page pointer to table definition |
| ???? | 4 bytes | num_rows | number of records on this page |
+------+---------+---------------------------------------------------------+
| Iterate for the number of records |
+--------------------------------------------------------------------------+
| ???? | 2 bytes | offset_row | The records location on this page |
+--------------------------------------------------------------------------+

In Jet4, an additional four byte field was added. It's purpose is currently
unknown.

+--------------------------------------------------------------------------+
| Jet4 Data Page Definition |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| 0x01 | 1 byte | page_type | 0x01 indicates a data page. |
| 0x01 | 1 byte | unknown | |
| ???? | 2 bytes | free_space | Free space in this page |
| ???? | 4 bytes | tdef_pg | Page pointer to table definition |
| ???? | 4 bytes | unknown | Unknown |
| ???? | 4 bytes | num_rows | number of records on this page |
+------+---------+---------------------------------------------------------+
| Iterate for the number of records |
+--------------------------------------------------------------------------+
| ???? | 2 bytes | offset_row | The records location on this page |
+--------------------------------------------------------------------------+

Notes for offset_row:
- Offsets that have 0x40 in the high order byte point to a location within
the page where a Data Pointer (4 bytes) to another data page is stored. Also
known as an overflow page.
- Offsets that have 0x80 in the high order byte are deleted rows.
(These flags are delflag and lookupflag in source code)

Rows are stored from the end of the page to the top of the page. So, the first
row stored runs from bytes offset_row to page_size - 1. The next row runs from
its offset to the previous row's offset, and so on.

Decoding a row requires knowing the number and types of columns from its TDEF
page. Decoding is handled by the routine mdb_crack_row().

The Jet3 row format is:

+--------------------------------------------------------------------------+
| Jet3 Row Definition |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| ???? | 1 byte | num_cols | Number of columns stored on this row. |
| ???? | n bytes | fixed_cols | Fixed length columns |
| ???? | n bytes | var_cols | Variable length columns |
| ???? | 1 byte | eod | length of data from begining of record |
| ???? | n bytes | var_table[]| offset from start of row for each var_col |
| ???? | 1 byte | var_len | number of variable length columns |
| ???? | n bytes | jump_table | number of variable length columns |
| ???? | n bytes | null_mask | Null indicator. size is 1 byte per 8 cols |
| | | | 0 indicates a null value. Also used to |
| | | | represent value of boolean type columns |
+--------------------------------------------------------------------------+

Notes:

. A row will always have the number of fixed columns as specified in the table
definition, but may have less variable columns, as rows are not updated when
columns are added.
. All fixed length columns are stored first to last, followed by variable length
columns.
. The size of the null table is computed by (num_cols - 1)/8 + 1
. Fixed columns can be null (unlike some other databases).
. The var_len field indicates the size of the var_table[].
. The eod field points at the last byte of the var_cols field. It is used to
determine where the last var_col ends.
. For boolean fixed columns, the values are in null_table[]: 0 indicates a false
value, 1 indicates a true value
. An 0xFF stored in the var_table indicates that this column has been deleted.

In Jet3 offsets are stored as 1 byte fields yielding a maximum of 256 bytes. To
get around this offsets are computed using a jump table. The jump table stores
the number of the first column in this jump segment. If the size of the data is
less than 256 then no jump table will be present.

For example if the row contains 45 columns and the offset of the 14th column is
more than 256 then the first entry in the jump table will be 0xe (14). If the
23rd column is the first one at offset > 512 the second entry of the jump table
would be 0x17 (23) and so on.

+--------------------------------------------------------------------------+
| Jet4 Row Definition |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| ???? | 2 bytes | num_cols | Number of columns stored on this row. |
| ???? | n bytes | fixed_cols | Fixed length columns |
| ???? | n bytes | var_cols | Variable length columns |
| ???? | 2 bytes | eod | length of data from begining of record |
| ???? | n bytes | var_table[]| offset from start of row for each var_col |
| ???? | 2 bytes | var_len | number of variable length columns |
| ???? | n bytes | null_mask | Null indicator. size is 1 byte per 8 cols |
| | | | 0 indicates a null value. Also used to |
| | | | represent value of bit type columns |
+--------------------------------------------------------------------------+

Notes:

. All offsets are stored as 2 byte fields including the var_table entries.
. the jump table was (thankfully) ditched in Jet4.

Each memo column (or other long binary data) in a row

+-------------------------------------------------------------------------+
| Memo Field Definition (12 bytes)
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| ???? | 2 bytes | memo_len | Total length of the memo |
| ???? | 2 bytes | bitmask | See values |
| ???? | 4 bytes | lval_dp | Data pointer to LVAL page (if needed) |
| 0x00 | 4 bytes | unknown | |
+------+---------+-------------+------------------------------------------+
Values for the bitmask:

0x8000= the memo is in a string at the end of this header (memo_len bytes)
0x4000= the memo is in a unique LVAL page in a record type 1
0x0000= the memo is in n LVAL pages in a record type 2

If the memo is in a LVAL page, we use row_id of lval_dp to find the row.
offset_start of memo = (int16*) LVAL_page[ 10 + row_id * 2]
if (rowid=0)
offset_stop of memo = 2048
else
offset_stop of memo = (int16*) LVAL_page[ 10 + row_id * 2 - 2]

The length (partial if type 2) for the memo is:
memo_page_len = offset_stop - offset_start

LVAL Pages
----------
(LVAL Page are particular data pages for long data storages )

The header of a LVAL page looks like this (10 bytes) :
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| 0x01 | 1 bytes | page_type | 0x01 indicate a data page |
| 0x01 | 1 bytes | unknown | |
| ???? | 2 bytes | free_space | The free space in this page |
| LVAL | 4 bytes | lval_id | The word 'LVAL' |
| ???? | 2 bytes | num_rows | Number of rows in this page |
+-------------------------------------------------------------------------+
| Iterate for the number of records |
+-------------------------------------------------------------------------+
| ???? | 2 bytes | row_offset | to the records location on this page |
+-------------------------------------------------------------------------+

Each memo record type 1 looks like this:
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| ???? | n bytes | memo_value | A string which is the memo |
+-------------------------------------------------------------------------+

Each memo record type 2 looks like this:
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| ???? | 4 bytes | lval_dp | Next page LVAL type 2 if memo is too long|
| ???? | n bytes | memo_value | A string which is the memo (partial) |
+-------------------------------------------------------------------------+

In a LVAL type 2 data page, you have
10 or 12 bytes for the header of the data page,
2 bytes for an offset,
4 bytes for the next lval_pg

So there is a bloc of 2048 - (10+2+4) = 2032(jet3)
or 4096 - (12+2+4) = 4078(jet4) bytes max in a page.

TDEF Pages (Table Definition)
-----------------------------

Every table in the database has a TDEF page. It contains a definition of
the columns, types, sizes, indexes, and similar information.

+-------------------------------------------------------------------------+
| Jet3/Jet4 TDEF Header
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| 0x02 | 1 bytes | page_type | 0x02 indicate a tabledef page |
| 0x01 | 1 bytes | unknown | |
| 'VC' | 2 bytes | tdef_id | The word 'VC' (Jet3 only, Jet4 unknown) |
| 0x00 | 4 bytes | next_pg | Next tdef page pointer (0 if none) |
+------+---------+-------------+------------------------------------------+

TDEFs can span multiple pages for large tables, this is accomplished using the
next_pg field.


+-------------------------------------------------------------------------+
| Jet3 Table Definition Block (35 bytes) |
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| ???? | 4 bytes | tdef_len | Length of the data for this page |
| ???? | 4 bytes | num_rows | Number of records in this table |
| 0x00 | 4 bytes | autonumber | value for the next value of the |
| | | | autonumber column, if any. 0 otherwise |
| 0x4e | 1 byte | table_type | 0x53: user table, 0x4e: system table |
| ???? | 2 bytes | max_cols | Max columns a row will have (deletions) |
| ???? | 2 bytes | num_var_cols| Number of variable columns in table |
| ???? | 2 bytes | num_cols | Number of columns in table (repeat) |
| ???? | 4 bytes | num_idx | Number of indexes in table |
| ???? | 4 bytes | num_real_idx| Number of indexes in table (repeat) |
| ???? | 4 bytes | used_pages | Points to a record containing the |
| | | | usage bitmask for this table. |
| ???? | 4 bytes | free_pages | Points to a similar record as above, |
| | | | listing pages which contain free space. |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx (8 bytes per idxs) |
+-------------------------------------------------------------------------+
| 0x00 | 4 bytes | ??? | |
| ???? | 4 bytes | num_idx_rows| (not sure) |
+-------------------------------------------------------------------------+
| Iterate for the number of num_cols (18 bytes per column) |
+-------------------------------------------------------------------------+
| ???? | 1 byte | col_type | Column Type (see table below) |
| ???? | 2 bytes | col_num | Column Number, (not always) |
| ???? | 2 bytes | offset_V | Offset for variable length columns |
| ???? | 4 bytes | ??? | |
| ???? | 4 bytes | ??? | |
| ???? | 1 byte | bitmask | low order bit indicates variable columns |
| ???? | 2 bytes | offset_F | Offset for fixed length columns |
| ???? | 2 bytes | col_len | Length of the column (0 if memo) |
+-------------------------------------------------------------------------+
| Iterate for the number of num_cols (n bytes per column) |
+-------------------------------------------------------------------------+
| ???? | 1 byte | col_name_len| len of the name of the column |
| ???? | n bytes | col_name | Name of the column |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx (30+9 = 39 bytes) |
+-------------------------------------------------------------------------+
| Iterate 10 times for 10 possible columns (10*3 = 30 bytes) |
+-------------------------------------------------------------------------+
| ???? | 2 bytes | col_num | number of a column (0xFFFF= none) |
| ???? | 1 byte | col_order | 0x01 = ascendency order |
+-------------------------------------------------------------------------+
| ???? | 4 bytes | unknown | |
| ???? | 4 bytes | first_dp | Data pointer of the index page |
| ???? | 1 byte | flags | See flags table for indexes |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx |
+-------------------------------------------------------------------------+
| ???? | 4 bytes | index_num | Number of the index |
| | | |(warn: not always in the sequential order)|
| ???? | 4 bytes | index_num2 | Number of the index (repeat) |
| 0xFF | 4 bytes | ??? | |
| 0x00 | 4 bytes | ??? | |
| 0x04 | 2 bytes | ??? | |
| ???? | 1 byte | primary_key | 0x01 if this index is primary |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx |
+-------------------------------------------------------------------------+
| ???? | 1 byte | idx_name_len| len of the name of the index |
| ???? | n bytes | idx_name | Name of the index |
+-------------------------------------------------------------------------+
| ???? | n bytes | ??? | |
| 0xFF | 2 bytes | ??? | End of the tableDef ? |
+-------------------------------------------------------------------------+

+-------------------------------------------------------------------------+
| Jet4 Table Definition Block (55 bytes) |
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| ???? | 4 bytes | tdef_len | Length of the data for this page |
| ???? | 4 bytes | unknown | unknown |
| ???? | 4 bytes | num_rows | Number of records in this table |
| 0x00 | 4 bytes | autonumber | value for the next value of the |
| | | | autonumber column, if any. 0 otherwise |
| ???? |16 bytes | unknown | unknown |
| 0x4e | 1 byte | table_type | 0x53: user table, 0x4e: system table |
| ???? | 2 bytes | max_cols | Max columns a row will have (deletions) |
| ???? | 2 bytes | num_var_cols| Number of variable columns in table |
| ???? | 2 bytes | num_cols | Number of columns in table (repeat) |
| ???? | 4 bytes | num_idx | Number of indexes in table |
| ???? | 4 bytes | num_real_idx| Number of indexes in table (repeat) |
| ???? | 4 bytes | used_pages | Points to a record containing the |
| | | | usage bitmask for this table. |
| ???? | 4 bytes | free_pages | Points to a similar record as above, |
| | | | listing pages which contain free space. |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx (12 bytes per idxs) |
+-------------------------------------------------------------------------+
| 0x00 | 4 bytes | ??? | |
| ???? | 4 bytes | num_idx_rows| (not sure) |
| 0x00 | 4 bytes | ??? | |
+-------------------------------------------------------------------------+
| Iterate for the number of num_cols (25 bytes per column) |
+-------------------------------------------------------------------------+
| ???? | 1 byte | col_type | Column Type (see table below) |
| ???? | 4 bytes | unknown | matches first unknown definition block |
| ???? | 2 bytes | col_num | Column Number |
| ???? | 2 bytes | offset_V | Offset for variable length columns |
| ???? | 2 bytes | col_num | Column Number (repeat) |
| ???? | 4 bytes | ??? | |
| ???? | 1 byte | bitmask | low order bit indicates variable columns |
| ???? | 1 byte | ??? | seems to be 1 when variable len |
| 0000 | 4 bytes | ??? | |
| ???? | 2 bytes | offset_F | Offset for fixed length columns |
| ???? | 2 bytes | col_len | Length of the column (0 if memo) |
+-------------------------------------------------------------------------+
| Iterate for the number of num_cols (n*2 bytes per column) |
+-------------------------------------------------------------------------+
| ???? | 2 bytes | col_name_len| len of the name of the column |
| ???? | n bytes | col_name | Name of the column (UCS-2 format) |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx (30+9 = 39 bytes) |
+-------------------------------------------------------------------------+
| ???? | 4 bytes | ??? | |
+-------------------------------------------------------------------------+
| Iterate 10 times for 10 possible columns (10*3 = 30 bytes) |
+-------------------------------------------------------------------------+
| ???? | 2 bytes | col_num | number of a column (0xFFFF= none) |
| ???? | 1 byte | col_order | 0x01 = ascendency order |
+-------------------------------------------------------------------------+
| ???? | 4 bytes | unknown | |
| ???? | 4 bytes | first_dp | Data pointer of the index page |
| ???? | 1 byte | flags | See flags table for indexes |
| ???? | 9 bytes | unknown | |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx (27 bytes) |
+-------------------------------------------------------------------------+
| ???? | 4 bytes | unknown | matches first unknown definition block |
| ???? | 4 bytes | index_num | Number of the index |
| | | |(warn: not always in the sequential order)|
| ???? | 4 bytes | index_num2 | Number of the index (repeat) |
| 0xFF | 4 bytes | ??? | |
| 0x00 | 4 bytes | ??? | |
| 0x04 | 2 bytes | ??? | |
| ???? | 1 byte | primary_key | 0x01 if this index is primary |
| ???? | 4 bytes | unknown | |
+-------------------------------------------------------------------------+
| Iterate for the number of num_real_idx |
+-------------------------------------------------------------------------+
| ???? | 2 bytes | idx_name_len| len of the name of the index |
| ???? | n bytes | idx_name | Name of the index (UCS-2) |
+-------------------------------------------------------------------------+
| ???? | n bytes | ??? | |
| 0xFF | 2 bytes | ??? | End of the tableDef ? |
+-------------------------------------------------------------------------+

Index flags (not complete):
0x01 Unique
0x02 IgnoreNuls
0x08 Required


Column Type may be one of the following (not complete):

BOOL = 0x01 /* boolean ( 1 bit ) */
BYTE = 0x02 /* byte ( 8 bits ) */
INT = 0x03 /* Integer (16 bits ) */
LONGINT = 0x04 /* Long Integer (32 bits ) */
MONEY = 0x05 /* Currency ( 8 bytes) */
FLOAT = 0x06 /* Single ( 4 bytes) */
DOUBLE = 0x07 /* Double ( 8 bytes) */
SDATETIME = 0x08 /* Short Date/Time ( 8 bytes) */
BINARY = 0x09 /* binay (255 bytes) */
TEXT = 0x0A /* Text (255 bytes) */
OLE = 0x0B /* OLE */
MEMO = 0x0C /* Memo, Hyperlink */
UNKNOWN_0D = 0x0D
REPID = 0x0F /* GUID */

Notes on deleted and added columns: (sort of Jet4 specific)

If a fixed length column is deleted the offset_F field will contain the offsets
of the original row definition. Thus is the number of columns on the row does
not match the number in the tdef, the offset_F field could be used to return
the proper data. Columns are never really deleted in the row data. The deleted column will forever exist and be set to null for new rows.

A row may have less than max_cols columns but will never have more, as max_cols
is never decremented. If you have a table with 6 columns, delete one, and add
one, then max_cols will be 7.

For variable length columns, offset_V will hold the position in the offset table
of that column. Missing columns are set to null for new rows.


Page Usage Maps
---------------

There are three uses for the page usage bitmaps. There is a global page usage
stored on page 1 which tracks allocated pages throughout the database.

Tables store two page usage bitmaps. One is straight map of which pages are
owned by the table. The second is a map of the pages owned by the table which
have free space on them (used for inserting data).

The table bitmaps appear to be of a fixed size for both Jet 3
and 4 (128 and 64 bytes respectively). The first byte of the map is a type
field.

Type 0 page usage map definition follows:

+--------------------------------------------------------------------------+
| Type 0 Page Usage Map |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| 0x00 | 1 byte | map_type | 0x00 indicates map stored within. |
| ???? | 4 byte | page_start | first page for which this map applies |
+------+---------+---------------------------------------------------------+
| Iterate for the length of map |
+--------------------------------------------------------------------------+
| ???? | 1 byte | bitmap | each bit encodes the allocation status of a|
| | | | page. 1 indicates allocated to this table. |
| | | | Pages are stored from msb to lsb. |
+--------------------------------------------------------------------------+

If you're paying attention then you'll realize that the relatively small size of
the map (128*8*2048 or 64*8*4096 = 2 Meg) means that this scheme won't work with
larger database files although the initial start page helps a bit. To overcome
this there is a second page usage map scheme with the map_type of 0x01 as
follows:

+--------------------------------------------------------------------------+
| Type 1 Page Usage Map |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| 0x01 | 1 byte | map_type | 0x01 indicates this is a indirection list. |
+------+---------+---------------------------------------------------------+
| Iterate for the length of map |
+--------------------------------------------------------------------------+
| ???? | 4 bytes | map_page | pointer to page type 0x05 containing map |
+--------------------------------------------------------------------------+

Note that the intial start page is gone and is reused for the first page
indirection. The 0x05 type page header looks like:

+--------------------------------------------------------------------------+
| Usage Map Page (type 0x05) |
+------+---------+---------------------------------------------------------+
| data | length | name | description |
+------+---------+---------------------------------------------------------+
| 0x05 | 1 byte | page_type | allocation map page |
| 0x01 | 1 byte | unknown | always 1 as with other page types |
| 0x00 | 2 bytes | unknown | |
+------+---------+---------------------------------------------------------+

The rest of the page is the allocation bitmap following the same scheme (lsb
to msb order, 1 bit per page) as a type 0 map. This yields a maximum of
2044*8=16352 (jet3) or 4092*8 = 32736 (jet4) pages mapped per type 0x05 page.
Given 128/4+1 = 33 or 64/4+1 = 17 page pointers per indirection row (remember
the start page field is reused, thus the +1), this yields 33*16352*2048 = 1053
Meg (jet3) or 17*32736*4096 = 2173 Meg (jet4) or enough to cover the maximum
size of each of the database formats comfortably, so there is no reason to
believe any other page map schemes exist.

Indices
-------

Indices are not completely understood but here is what we know.

+-------------------------------------------------------------------------+
| Index Page (type 0x03) |
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| 0x01 | 1 bytes | page_type | 0x03 indicate an index page |
| 0x01 | 1 bytes | unknown | |
| ???? | 2 bytes | free_space | The free space at the end this page |
| ???? | 4 bytes | parent_page | The page number of the TDEF for this idx |
| ???? | 4 bytes | prev_page | Previous page at this index level |
| ???? | 4 bytes | next_page | Next page at this index level |
| ???? | 4 bytes | leaf_page | Pointer to leaf page, purpose unknown |
+-------------------------------------------------------------------------+

Index pages come in two flavors.

0x04 pages are leaf pages which contain one entry for each row in the table.
Each entry is composed of a flag, the indexed column values and a page/row
pointer to the data.

0x03 index pages make up the rest of the index tree and contain a flag, the
indexed columns, the page/row contains this entry, and the leaf page or
intermediate (another 0x03 page) page pointer for which this is the first
entry on.

Both index types have a bitmask starting at 0x16 which identifies the starting
location of each index entry on this page. The first entry is assumed and
the count starts from the low order bit. For example take the data:

00 20 00 04 80 00 ...

This first entry starts at 0xf8 (always). Convert the bytes to binary starting
with the low order bit and stopping at the first "on" bit:

0000 0000 0000 01
-- 00 --- -- 20 -->
This next entry starts 14 (0xe) bytes in at 0x105. Proceding from here, the next
entry:

00 0000 0000 001
<-- 20 -- -- 00 --- -- 04

starts 13 (0xd) bytes further in at 0x112. The final entry starts at

0 0000 0000 0001
<-- 04 -- -- 80 ---

or 13 (0xd) bytes more at 0x120. In this example the rest of the mask (up
to offset 0xf8) would be zero filled and thus this last entry at 0x120 isn't
an actual entry but the stopping point of the data.

Since 0xf8 = 248 and 0x16 = 22, (248 - 22) * 8 = 1808 and 2048 - 1808 = 240
leaving just enough space for the bit mask to encode the remainder of the page.
One wonders why MS didn't use a row offset table like they did on data pages,
seems like it would have been easier and more flexible.

So now we come to the index entries for type 0x03 pages which look like this:

+-------------------------------------------------------------------------+
| Index Record |
+------+---------+-------------+------------------------------------------+
| data | length | name | description |
+------+---------+-------------+------------------------------------------+
| 0x7f | 1 byte | flags | 0x80 LSB, 0x7f MSB, 0x00 null? |
| ???? | variable| indexed cols| indexed column data |
| ???? | 3 bytes | data page | page containing row referred to by this |
| | | | index entry |
| ???? | 1 byte | data row | row number on that page of this entry |
| ???? | 4 bytes | child page | next level index page containing this |
| | | | entry as first entry. Could be a leaf |
| | | | node. |
+-------------------------------------------------------------------------+

The flag field is generally either 0x00, 0x7f, 0x80. 0x80 is the one's
complement of 0x7f and all text data in the index would then need to be negated.
The reason for this negation is unknown, although I suspect it has to do with
descending order. The 0x00 flag indicates that the key column is null, and no
data will follow, only the page pointer. In multicolumn indexes the flag field
plus data is repeated for the number of columns participating in the key.

Update: There is a compression scheme utilized on leaf pages as follows:
Normally an index entry with an integer primary key would be 9 bytes (1
for the flags field, 4 for the integer, 3 for page, and 1 for row). The
entry can be shorter than 9, containing only 5 bytes, the first byte is the last
octet of the encoded primary key field (integer) and the last four are the page/row
pointer. Thus if the first key value on the page is 1 and it points to page 261
(00 01 05) row 3, it becomes
7f 00 00 00 01 00 01 05 03

the next index entry can be:
02 00 01 05 04

that is, the key value is 2 (the last octet changes to 02) page 261 row 4.

Access stores an 'alphabetic sort order' version of the text key columns in the index.
Basically this means that upper and lower case characters A-Z are merged and start at
0x60. Digits are 0x56 through 0x5f. Once converted into this (non-ascii) character set,
the text value is able to be sorted in 'alphabetic' order. A text column will end with
a NULL (0x00 or 0xff if negated).

The leaf page entries store the key column and the 3 byte page and 1 byte row
number.

The value of the index root page in the index definition may be an index page
(type 0x03), an index leaf page (type 0x04) if there is only one index page,
or (in the case of tables small enough to fit on one page) a data page
(type 0x01).

So to search the index, you need to convert your value into the alphabetic
character set, compare against each index entry, and on successful comparison
follow the page and row number to the data. Because text data is managled
during this conversion there is no 'covered querys' possible on text columns.

To conserve on frequent index updates, Jet also does something special when
creating new leaf pages at the end of a primary key (maybe others as well) index.
The next leaf page pointer of the last leaf node points to the new leaf page but
the index tree is not otherwise updated. In src/libmdb/index.c, the last leaf
read is stored, once the index search has been exhausted by the normal search
routine, it enters a "clean up mode" and reads the next leaf page pointer until
it's null.

KKD Records
-----------

Design View table definitions appear to be stored in 'KKD' records (my name for
them...they always start with 'KKD\0'). Again these reside on pages, packed to
the end of the page.

Update: The KKD records are stored in LvProp column of MSysObjects so they are
stored as other OLE/Memo fields are.

They look a little like this:
'K' 'K' 'D' 0x00
16 bit length value (this includes the length)
0x00 0x00
0x80 0x00 (0x80 seems to indicate a header)
Then one or more of: 16 bit length field and a value of that size.
For instance:
0x0d 0x00 and 'AccessVersion' (AccessVersion is 13 bytes, 0x0d 0x00 intel order)

Next comes one of more rows of data. (column names, descriptions, etc...)
16 bit length value (this includes the length)
0x00 0x00
0x00 0x00
16bit length field (this include the length itself)
4 bytes of unknown purpose
16 bit length field (non-inclusive)
value (07.53 for the AccessVersion example above)

 






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