Consider an IFD entry with a rational type (UNSIGNED RATIONAL or SIGNED RATIONAL).
A value that has a rational type is 8 bytes.
The "value offset" field is 4 bytes.
Therefore, the "value offset" field of an IFD entry with a rational type can only hold an offset (the field can never hold a value).
Is that correct?
Short version: Yes.
Long version:
The TIFF IFD consists of IFD entries, which again consist of 12 bytes, containing, in order:
2 byte Tag identifier
2 byte Type
4 byte Count
4 byte Value OR Offset
Quoting from the TIFF 6.0 spec:
Value/Offset
To save time and space the Value Offset contains the Value instead of pointing to
the Value if and only if the Value fits into 4 bytes. If the Value is shorter than 4
bytes, it is left-justified within the 4-byte Value Offset, i.e., stored in the lowernumbered
bytes. Whether the Value fits within 4 bytes is determined by the Type
and Count of the field.
So, as the rational types are always at least 8 bytes long, they can never appear as an "inline" value, only as offsets + value somewhere else in the file.
Related
I've been reading about disc usage/space for different strings, it says that LONGTEXT takes 4GB.
Is that disk space declared for FULLY FILLED column or JUST CREATED (Empty)
Thank You.
The answer is: L + 4 bytes, where L < 2^32
Variable-length string types are stored using a length prefix plus
data. The length prefix requires from one to four bytes depending on
the data type, and the value of the prefix is L (the byte length of
the string). For example, storage for a MEDIUMTEXT value requires L
bytes to store the value plus three bytes to store the length of the
value.
Source: https://dev.mysql.com/doc/refman/5.7/en/storage-requirements.html#data-types-storage-reqs-strings
I understood that in a database an int takes less space than a string. But what if the int is really longer than the string. For example 9.455.487 vs "John". Which one will take more space? TY
From the documentation, size of int is 4 bytes, whereas for char it is "M × w bytes, 0 <= M <= 255, where w is the number of bytes required for the maximum-length character in the character set." and M is the declared column size.
So when you talk of how much space is taken, the int will take up 4 bytes for a value as long as the value is within the range of int. A string like "John", if declared as char(4) will take up 4 * w bytes, so at least 4 bytes assuming w is 1.
Long story short, the size of a number is not how many characters long it is when you write it out, but the number of bytes to represent it in the binary form.
You should be aware of what "int" (integer) is and what strings are. Integer always has some length and that length means how many bytes are in it's binary representation. On the other hand, strings are sequences of bytes. So, depending of encoding, each symbol may be one or more bytes.
The thing that 9.455.487 is "longer" than "John" is irrelevant here. What is relevant - is how DBMS (or whatever other environment) will represent those things. You're seeing "longer" integer versus "shorter" string while it's not so, it's only a matter of "screen" representation (i.e. what you see on the screen).
Answering question - for MySQL, INT is 4 bytes, while string data types may have dynamic length - such as VARCHAR. Static string length date type is CHAR and from that viewpoint, your number and your string will have same length (4 bytes). Strings and integers are just different things to compare for "length". And visual representation should not confuse you. This entities have different internal structure, and, therefore, should not be compared on "length" according to their visual representation.
Also, you should be aware that not always integer will have 4 bytes length - since even in MySQL your number may belong to, for example, BIGINT data type (which is 8 bytes length). And, as mentioned above, for strings there's also encoding issue. For instance, UTF-8 encoded string may have two (or even more) bytes to represent some non-ASCII symbols. In this case each symbol will add more that 1 byte to total string length.
I was reading about the MySQL data type size. I saw VARCHAR takes extra 1/2 bytes, MEDIUMTEXT requires extra 3 bytes, LONGTEXT requires extra 4 bytes. What is the reason for such MySQL behaviour?
When MySQL (or any database or computer language) stores a variable length string, there are basically two ways to store the value:
The length can be encoded followed by the characters in the string
The end of the string can be marked by a special character (typically '0')
Databases (almost?) always use length encoding. So, when you store 'ABC' as a variable length string, in the database storage it looks like:
3 A B C
When you store 'A':
1 A
That way, MySQL knows when one string ends and the next begins. The different lengths for the different types are based on the maximum length of the string. So, 1 byte can hold values from 0 to 255. 2 bytes can hold values from 0 to 65,535 and so on.
When you use a regular character expression, say char(3), then 'ABC' looks like:
A B C
This occupies three bytes/whatever (depending on the character coding). The length is known from the table metadata.
With char(3), the string 'A' also occupies three slots:
A
---^space here
--------^space here
The extra two are occupied by spaces. For long strings, this is generally a big waste of space, which is why most strings are stored as varchar rather than char.
This is probably a stupid question, but i need to ask...
I've created a MySQL table to handle images called images. In it, I have an attribute that keeps the extension of the image called extension.
Most of the accepted images extensions are either jpg or png or gif or bmp or jpeg or tiff In other words, a maximum of 4 characters long.
Now, should the attribute be declared in the MySQL table like:
extension char(4)
or
extension varchar(4)
There's probably no impact what so ever on performance, but i do want the model to be optimize from the get go...
Anyone?
Depends....
If you look at this from the MySQL documentation
Value CHAR(4) Storage Required VARCHAR(4) Storage Required
'' ' ' 4 bytes '' 1 byte
'ab' 'ab ' 4 bytes 'ab' 3 bytes
'abcd' 'abcd' 4 bytes 'abcd' 5 bytes
'abcdefgh' 'abcd' 4 bytes 'abcd' 5 bytes
As you can see 4 characters for CHAR takes 4 bytes, while VARCHAR takes 5. If the vast majority of extensions would be 4 characters then CHAR would be more space efficient.
In your case I am guessing that 3 will be majority so VARCHAR is the better choice.
James :-)
Edited, I was making a wrong assumption on my previous answer. I'll just paste you an excerpt from http://dev.mysql.com/doc/refman/5.0/en/char.html (emphasis added)
The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved. As of MySQL 5.0.3, they also differ in maximum length and in whether trailing spaces are retained.
The CHAR and VARCHAR types are declared with a length that indicates the maximum number of characters you want to store. For example, CHAR(30) can hold up to 30 characters.
The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value from 0 to 255. When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.
Values in VARCHAR columns are variable-length strings. The length can be specified as a value from 0 to 255 before MySQL 5.0.3, and 0 to 65,535 in 5.0.3 and later versions. The effective maximum length of a VARCHAR in MySQL 5.0.3 and later is subject to the maximum row size (65,535 bytes, which is shared among all columns) and the character set used.
In contrast to CHAR, VARCHAR values are stored as a one-byte or two-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes.
I'm using MySQL, all my tables are using InnoDB engine. I have some columns declared as DECIMAL(38, 0) and they are used extensively. According to the MySQL documentation (http://dev.mysql.com/doc/refman/5.5/en/storage-requirements.html), 38-digit value requires 17 bytes (38 = 4 * 9 + 2; 4 * 4 + 1 = 17). Okay.
But, does that mean that any value stored in this column will take 17 bytes? For example, for value 432 - will it take 4 bytes only (I really hope so...) or will it take 17 bytes anyway?
Finally, I know that in Oracle the size occupied depends on the actual values stored. But is it optimized that way in MySQL as well?
I think the answer is that it will take 17 bytes anyway. If you notice, detailed in the linked manual page there is no means for the DBMS to record how "long" the value is. By comparison, for a VARCHAR(255) CHARACTER SET ascii column there is a single byte at the start of the value that indicates how long the value is (for a maximum size of 256 bytes). For a VARCHAR(1000) CHARACTER SET ascii column there are two bytes to indicate the length. Here no means is detailed to record the length of the value, leading me to conclude that the column always takes the maximum amount of space.
Decimal is "fixed length" so every value requires 17 bytes