I have a MySQL table with one of the columns like "varchar(255)". Will the Database use 255bytes of space even if that column is empty? or only if it has some data and the amount of space used is proportional to the data?
every cell will take only the amount of space proportional to the data.
http://dev.mysql.com/doc/refman/5.0/en/char.html
only if it has some data and the amount of space used is proportional to the data.
VARCHAR(M) takes N+1 bytes or more depending on the size of the data you're adding. A blank field (empty string) will still consume 1 byte (that is the +1 on the N+1) that is used to indicate where that field's data ends - so you have 1 byte for the terminator.
From MySQL's website:
VARCHAR(M), VARBINARY(M)
L + 1 bytes if column values require 0 – 255 bytes,
L + 2 bytes if values may require more than 255 bytes
where L is the length of your data. In your case, you'll be consuming (data length + 1) on your VARCHAR(255) field.
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 have this table, which contains a column, which can store strings of different sizes, these can be simple words like "hello" or long texts of up to 25 thousand characters.
I know the byte sizes of the data types and I have read some answers from this same site, but I have not found concrete references that allow me to decide on this particular case.
25000 maximum data is too much for varchar?
Maybe yes, then I should use text. But what if most of the strings do not exceed 20 characters and there are only a few exceptions where the text is 25000 characters long?
What type of data should I use? Varchar (25000) or text?
If you're not building something with has to take care of every little byte of space on your DB-server I guess it doesn't really matter. If you have a large amout of records below 255 bytes, you'll save about one byte for each record if going for varchar.
In cases like this I personally prefer text, mainly because avoiding running into trouble with a too small defined length.
From the MySQL Documentation:
Data Type: VARCHAR(M), VARBINARY(M)
Storage Required: L + 1 bytes if column values require 0 − 255 bytes, L + 2 bytes if values may require
more than 255 bytes
Data Type: BLOB, TEXT
Storage Required: L + 2 bytes, where L < 2^16
I run a website with MySql db back end. I need to know what is the impact when we choose a column type (say MediumTEXT) in order to save some heavy data.
eg :
MEDIUMTEXT | 16,777,215 (224−1) bytes = 16 MiB
From above , the MEDIUMTEXT is 16,777,215 (224−1) bytes . which means it can hold up to 16Mb of data. Does this mean, it reserves 16mb data to every entry inserted?
i.e if my entry is just "Hello World", how would mysql ( or in the case any db) handle writing to the disc?
No, it doesn't reserve all that space. It allocates 3 bytes per row to store the (future, variable) length of the actual data, and then only as much space is used as is needed to store the data for each row created.
L represents the actual length in bytes of a given string value.
...storage for a MEDIUMTEXT value requires L bytes to store the value plus three bytes to store the length of the value.
http://dev.mysql.com/doc/refman/5.6/en/storage-requirements.html
I am storing strings and 99.5+% are less than 255 characters, so I store them in a VARCHAR(255).
The thing is, some of them can be 4kb or so. What's the best way to store those?
Option #1: store them in another table with a pointer to the main.
Option #1.0: add an INT column with DEFAULT NULL and the pointer will be stored there
Option #1.1: the pointer will be stored in the VARCHAR(255) column, e.g 'AAAAAAAAAAA[NUMBER]AAAAAAAAAAAA'
Option #2: increase the size of VARCHAR from 255 to 32767
What's the best of the above, Option #1.0, Option #1.1 or Option #2, performance wise?
Increase the size of your field to fit the max size of your string. A VARCHAR will not use the space unless needed.
VARCHAR values are stored as a 1-byte or 2-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.
http://dev.mysql.com/doc/refman/5.0/en/char.html
The MySQL Definition says that VARCHAR(N) will take up to L + 1 bytes if column values require 0 – 255 bytes, L + 2 bytes if values may require more than 255 bytes where L is the length in bytes of the stored string.
So I guess that option #2 is quite okay, because the small strings will still take less space than 32767 bytes.
EDIT:
Also imagine the countless problems options 1.0 and 1.1 would raise when you actually want to query a string without knowing whether it exceeds the length or not.
Option #2 is clearly best. It just adds 1 byte to the size of each value, and doesn't require any complicated joins to merge in the fields from the second table.
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