When you say:
CREATE TABLE Table1 ...
MySQL creates files named
Table1.MYI ("MySQL Index"),
Table1.frm ("Format"). These files will
be in the directory:
For example, if you use Linux, you might find the files in the
/usr/local/var/test directory (assuming
your database name is
test). if you use
Windows, you might find the files in the
Let's look at the
.MYD Data file
MyISAM SQL Data file) more closely. There
are three possible formats — fixed, dynamic, and packed.
First, let's discuss the fixed format.
Unlike most DBMSs, MySQL doesn't store on disk using pages.
Therefore you will not see filler space between rows.
(Reminder: This does not refer to
InnoDB tables, which do use pages).
The minimal record header is a set of flags:
"X bit" = 0 if row is deleted, = 1 if row is not deleted
"Null Bits" = 1 if row contains any null fields, or = 0 otherwise.
"Filler Bits" = 1
The length of the record header is thus:
(1 + number of NULL columns + 7) / 8 bytes
After the header, all columns are stored in the order that they
were created, which is the same order that you would get from
Here's an example. Suppose you say:
CREATE TABLE Table1 (column1 CHAR(1), column2 CHAR(1), column3 CHAR(1)); INSERT INTO Table1 VALUES ('a', 'b', 'c'); INSERT INTO Table1 VALUES ('d', NULL, 'e');
CHAR(1) column takes precisely one byte
(plus one bit of overhead that is assigned to every column
— I'll describe the details of column storage later). So
Table1.MYD looks like this:
Hexadecimal Display of
F1 61 62 63 00 F5 64 00 66 00 ... .abc..d e.
Here's how to read this hexadecimal-dump display:
The hexadecimal numbers
F1 61 62 63 00 F5 64 20 65
00 are byte values and the column on the right is
an attempt to show the same bytes in ASCII.
F1 byte means that there are no null
fields in the first row.
F5 byte means that the second column
of the second row is
(It's probably easier to understand the flag setting if you
binary, and (a) notice that the third flag bit from
the right is
on, and (b) remember that the
first flag bit is the X bit.)
There are complications — the record header is more complex if there are variable-length fields — but the simple display shown in the example is exactly what you'd see if you looked at the MySQL Data file with a debugger or a hexadecimal file dumper.
So much for the fixed format. Now, let's discuss the dynamic format.
The dynamic file format is necessary if rows can vary in size.
That will be the case if there are
columns, or "true"
VARCHAR columns. (Remember
that MySQL may treat
VARCHAR columns as if
CHAR columns, in which case the fixed
format is used.) A dynamic row has more fields in the header.
The important ones are "the actual length", "the unused length",
and "the overflow pointer". The actual length is the total
number of bytes in all the columns. The unused length is the
total number of bytes between one physical record and the next
one. The overflow pointer is the location of the rest of the
record if there are multiple parts.
For example, here is a dynamic row:
03 start of header - Block type, see mi_dynrec.c, _mi_get_block_info() 04, 00 actual length 0c unused length 01, fc flags + overflow pointer **** data in the row ************ unused bytes <-- next row starts here)
In the example, the actual length and the unused length are short (one byte each) because the table definition says that the columns are short — if the columns were potentially large, then the actual length and the unused length could be two bytes each, three bytes each, and so on. In this case, actual length plus unused length is 10 hexadecimal (sixteen decimal), which is a minimum.
In a dynamic row, there is no deleted bit. Instead, deleted rows are marked with a block of type 0.
As for the third format — packed — we will only say briefly that:
Numeric values are stored in a form that depends on the range (start/end values) for the data type.
All columns are packed using either Huffman or enum coding.
For details, see the source files
/myisam/mi_statrec.c (for fixed format),
/myisam/mi_dynrec.c (for dynamic format),
/myisam/mi_packrec.c (for packed
Note: Internally, MySQL uses a format much like the fixed format which it uses for disk storage. The main differences are:
BLOB values have a length and a memory
pointer rather than being stored inline.
VARCHAR" (a column storage which
will be fully implemented in version 5.0) will have a 16-bit
length plus the data.
All integer or floating-point numbers are stored with the
low byte first. Point (3) does not apply for
ISAM storage or internals.
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