I have been thinking about database design lately and I have the following question:
When a type, say a varchar(max), is set for a column is 2GB of space set aside every time a row is inserted?
Or is the space allocated on the server equal to the amount of data in the column?
Thanks!
The varchar data type in SQL Server and elsewhere means roughly variable-length character data. The max or any other constant value represents its upper bound and not its absolute size. So your latter observation is correct:
the space allocated on the server
equal to the amount of data in the
column
Now, if you define something like char(200) (notice the lack of var in front of char there) then yes, 200 characters are allocated regardless of how much data (up to 200 chars) you store in that field. The maximum upper bound for the char data type is 8000, by the way.
Of course not :)
Varchar(max) is a bastardised hybrid data type (if you do not mind me being a bit frank):
It is stored with the rest of the columns in the row if total row length does not exceed 8KB limit
Otherwise stored as a blob. A blob (TEXT,IMAGE) takes as much as its length.
Related
I have a question in regards to data types that are available in SQL language to store data into the database itself. Since I'm dealing with database that is quite large, and has a tendency to expand over 150GB+ of data, I need to pay close attention and save up every bit of space on the server's hard drive so that the database doesn't takes up all the precious space. So my question is as following:
Which data type is the best to store 80-200 character long string in database?
I'm aware of for example varchar(200) and nvarchar(200) where the nvarchar supports unicode character. Which one of these would take up less space in database, or if there's a 3rd data type that I'm not aware of, and which I could use to store the data (if I know for a fact that the string I would store is just a combination of numbers and letters, without any special characters)
Are there some other techniques that I could use to save up space in database so that it doesn't expands rapidly ?
Can someone help me out with this ?
P.S. Guys, I have a 4th question as well:
If for example I have nvarchar(max) data type which is in a table, and the entered record takes up only 100 characters, how much data is reserved for that kind of record?
Let's say that I have ID which is of following form 191697193441 ... Would it make more sense to store this number as varchar(200) or bigint ?
The size needed for nvarchar is 2 bytes per character, as it represents unicode data. varchar needs 1 byte per character. The storage size is the actual number of characters entered + 2 bytes overhead. This is also true for varchar(max).
From https://learn.microsoft.com/en-us/sql/t-sql/data-types/char-and-varchar-transact-sql:
varchar [ ( n | max ) ] Variable-length, non-Unicode string data. n defines the string length and can be a value from 1 through 8,000. max indicates that the maximum storage size is 2^31-1 bytes (2 GB). The storage size is the actual length of the data entered + 2 bytes.
So for your 4th question, nvarchar would need 100 * 2 + 2 = 202 bytes, varchar would need 100 * 1 + 2 = 102 bytes.
There's no performance or data size difference as they're variable length data types, so they'll only use the space they need.
Think of the size parameter as more of a useful constraint. For e.g. if you have a surname field, you can reasonably expect 50 characters to be a sensible maximum size and you have more chance of a mistake (misuse of the field, incorrect data capture etc.) throwing an error, rather than adding nonsense to the database and needing future data cleansing.
So, my general rule of thumb is make them as large as the business requirements demand, but no larger. It's trivial to amend variable data sizes to a larger length in the future if requirements change.
I'm having trouble understanding how to define a column for my text that has the right size for my max. number of characters. In Oracle I can create a VARCHAR2(10 CHAR) which will be big enough for 10 characters. The size depends on the encoding used in the database. But how do I do that in SQL Server? Do I use varchar(10)? nvarchar(10)? I want to be able to store all kinds of characters (even chinese).
If you want Chinese characters, you need to use nvarchar(n) and specify a length of n that makes sense.
Those are characters you're defining, and the space you need is twice that number (since any Unicode character in SQL Server always uses 2 bytes).
Max. size is nvarchar(4000) - or if you really need more, use nvarchar(max) (for up to 1 billion characters).
I would recommend NOT to just use nvarchar(max) for everything, out of lazyness about considering what size you really need! Since it's a really large column, you won't be able to index it for one.
If you use nvarchar(max) this will allow for any number of characters for all character sets. The system will optimise storage.
Limitations on row size are addressed here. See answer from #marc_s for limitations on the use of max.
I have some data which I will be putting in the database. Say I make a field like "coupondetail text(10000)" which will store the coupon detail, now consider that not all coupondetail will be 10,000 chars long. I m curious to know how much space will the column take in the database when the coupondetail text is lesser than 10,000 say 1000 chars?
sqlite does not care much how you declare your column types and ignores any maximum length specified. The declared type is just a hint; any non-INTEGER PRIMARY KEY column can contain any type.
The size taken up in the database file depends on the values you put in. In the record format, strings are stored as length followed by string data. No empty space is necessarily left there.
I have a large table with say 10 columns. 4 of them remains null most of the times. I have a query that does null value takes any size or no size in bytes. I read few articles some of them are saying :
http://www.sql-server-citation.com/2009/12/common-mistakes-in-sql-server-part-4.html
There is a misconception that if we have the NULL values in a table it doesn't occupy storage space. The fact is, a NULL value occupies space – 2 bytes
SQL: Using NULL values vs. default values
A NULL value in databases is a system value that takes up one byte of storage and indicates that a value is not present as opposed to a space or zero or any other default value.
Can you please guide me regarding the size taken by null value.
If the field is fixed width storing NULL takes the same space as any other value - the width of the field.
If the field is variable width the NULL value takes up no space.
In addition to the space required to store a null value there is also an overhead for having a nullable column. For each row one bit is used per nullable column to mark whether the value for that column is null or not. This is true whether the column is fixed or variable length.
The reason for the discrepancies that you have observed in information from other sources:
The start of the first article is a bit misleading. The article is not talking about the cost of storing a NULL value, but the cost of having the ability to store a NULL (i.e the cost of making a column nullable). It's true that it costs something in storage space to make a column nullable, but once you have done that it takes less space to store a NULL than it takes to store a value (for variable width columns).
The second link seems to be a question about Microsoft Access. I don't know the details of how Access stores NULLs but I wouldn't be surprised if it is different to SQL Server.
The following link claims that if the column is variable length, i.e. varchar then NULL takes 0 bytes (plus 1 byte is used to flag whether value is NULL or not):
How does SQL Server really store NULL-s
The above link, as well as the below link, claim that for fixed length columns, i.e. char(10) or int, a value of NULL occupies the length of the column (plus 1 byte to flag whether it's NULL or not):
Data Type Performance Tuning Tips for Microsoft SQL Server
Examples:
If you set a char(10) to NULL, it occupies 10 bytes (zeroed out)
An int takes 4 bytes (also zeroed out).
A varchar(1 million) set to NULL takes 0 bytes (+ 2 bytes)
Note: on a slight tangent, the storage size of varchar is the length of data entered + 2 bytes.
From this link:
Each row has a null bitmap for columns
that allow nulls. If the row in that
column is null then a bit in the
bitmap is 1 else it's 0.
For variable size datatypes the
acctual size is 0 bytes.
For fixed size datatype the acctual
size is the default datatype size in
bytes set to default value (0 for
numbers, '' for chars).
Storing a NULL value does not take any space.
"The fact is, a NULL value occupies
space – 2 bytes."
This is a misconception -- that's 2 bytes per row, and I'm pretty sure that all rows use those 2 bytes regardless of whether there's any nullable columns.
A NULL value in databases is a system
value that takes up one byte of
storage
This is talking about databases in general, not specifically SQL Server. SQL Server does not use 1 byte to store NULL values.
Even though this questions is specifically tagged as SQL Server 2005, being that it is now 2021, it should be pointed out that it is a "trick question" for any version of SQL Server after 2005.
This is because if either ROW or PAGE compression are used, or if the column is defined as SPARSE, then it will "no space" in the actual row to store a 'NULL value'. These were added in SQL Server 2008.
The implementation notes for ROW COMPRESSION (which is a prerequisite for PAGE COMPRESSION) states:
NULL and 0 values across all data types are optimized and take no bytes1.
While there is still minimal metadata (4 bits per column + (record overhead / columns)) stored per non-sparse column in each physical record2, it's strictly not the value and is required in all cases3.
SPARSE columns with a NULL value take up no space and no relevant per-row metadata (as the number of SPARSE columns increase), albeit with a trade-off for non-NULL values.
As such, it is hard to "count" space without anlyzing the actual DB usage stats. The average bytes per row will vary based on precise column types, table/index rebuild settings, actual data and duplicity, fill capacity, effective page utilization, fragmentation, LOB usage, etc. and is often a more useful metric.
1 SQLite uses a similar approach to have effectively-free NULL values.
2 A brief of the technical layout used in ROW (and thus PAGE) compression can found in "SQL Server 2012 Internals: Special Storage".
Following the 1 or 2 bytes for the number of columns is the CD array, which uses 4 bits [of metadata] for each column in the table to represent information about the length of the column .. 0 (0×0) indicates that the corresponding column is NULL.
3 Fun fact: with ROW compression, bit column values exist entirely in the corresponding 4-bit metadata.
In sql server does it make a difference if I define a varchar column to be of length 32 or 128?
A varchar is a variable character field. This means it can hold text data to a certain length. A varchar(32) can only hold 32 characters, whereas a varchar(128) can hold 128 characters. If I tried to input "12345" into a varchar(3) field; this is the data that will be stored:
"123"
The "45" will be "truncated" (lost).
They are very useful in instances where you know that a certain field will only be (or only should be) a certain length at maximum. For example: a zip code or state abbreviation. In fact, they are generally used for almost all types of text data (such as names/addresses/et cetera) - but in these instances you must be careful that the number you supply is a sane maximum for the type of data that will fill that column.
However, you must also be careful when using them to only allow the user to input the maximum amount of characters that the field will support. Otherwise it may lend to confusion when it truncates the user's input.
There should be no noticeable difference as the backend will only store the amount of data you insert into that column. It's not padded out to the full size of the field like it is with a char column.
Edit: For more info, this link should be useful.
It should not. It just defines the maximum length it can accommodate, the actual used length depends on the data inserted.