Does such a thing exist or are you always required to set the max field length manually?
We have a varchar field in our database where the length of the value can be anything from 0 all the way to 800 or greater. Is there a datatype that will automatically set itself to the length of the value being entered?
(this is specifically for SQL Server - but should apply to most other RDBMS as well)
You need to define the maximum length - but the varchar type will always only store as much data as needed. So need to define the max - but not the actual size - that's handled automatically.
So if you have varchar(800) and store 50 characters - it takes up 52 bytes. If you store 100 characters - 102 bytes used (2 bytes overhead per entry).
At the same time, I would still argue you should NOT just define all column varchar(max) (2 GB max. size) - even though it might look tempting and convenient. Read What's the Point of Using VARCHAR(n) Anymore? for a great explanation of why not to do that.
Related
Should I define a column type from actual length to nth power of 2?
The first case, I have a table column store no more than 7 charactors,
will I use NVARCHAR(8)? since there maybe implicit convert inside Sql
server, allocate 8 space and truncate automatic(heard some where).
If not, NCHAR(7)/NCHAR(8), which should be(assume the fixed length is 7)
Any performance differ on about this 2 cases?
You should use the actual length of the string. Now, if you know that the value will always be exactly 7 characters, then use CHAR(7) rather than VARCHAR(7).
The reason you see powers-of-2 is for columns that have an indeterminate length -- a name or description that may not be fixed. In most databases, you need to put in some maximum length for the varchar(). For historical reasons, powers-of-2 get used for such things, because of the binary nature of the underlying CPUs.
Although I almost always use powers-of-2 in these situations, I can think of no real performance differences. There is one. . . in some databases the actual length of a varchar(255) is stored using 1 byte whereas a varchar(256) uses 2 bytes. That is a pretty minor difference -- even when multiplied over millions of rows.
In HANA, there's a column of type NVARCHAR(4000) with value ThisISaString, is the RAM that is being used = 4000 or 13?
If it reserves 4000, then this space could really add up when you have a lot of records.
I am trying to decide how big I should make my text fields.
What I make of your question in its current form is how SAP HANA handles variable length strings when it comes to presenting it to the client (I take from your intention to reserve a buffer.
Thus, I'm not going to discuss what happens inside of HANA when you enter a value into a table - this is rather complex and depends on the table type used (column, row, external, temporary...)
So, for the client application, a (N)VARCHAR
will result in a string with the length of the stored value, i.e. no padding (with spaces at the end) will happen.
You often see database fields set to have a magnitude of 255 characters, what is the traditional / historic reason why? I assume it's something to do with paging / memory limits, and performance but the distinction between 255 and 256 has always confused me.
varchar(255)
Considering this is a capacity or magnitude, not an indexer, why is 255 preferred over 256? Is a byte reserved for some purpose (terminator or null or something)?
Presumably varchar(0) is a nonsense (has zero capacity)? In which case 2^8 of space should be 256 surely?
Are there other magnitudes that provide performance benefits? For example is varchar(512) less performant than varchar(511) or varchar(510)?
Is this value the same for all relations databases, old and new?
disclaimer - I'm a developer not a DBA, I use field sizes and types that suit my business logic where that is known, but I'd like to know the historic reason for this preference, even if it's no longer relevant (but even more if it still is relevant).
Edit:
Thanks for the answers, there seems to be some concensus that a byte is used to store size, but this doesn't settle the matter definitively in my mind.
If the meta data (string length) is stored in the same contiguous memory/disk, it makes some sense. 1 byte of metadata and 255 bytes of string data, would suit each other very nicely, and fit into 256 contiguous bytes of storage, which presumably is neat and tidy.
But...If the metadata (string length) is stored separately from the actual string data (in a master table perhaps), then to constrain the length of string's data by one byte, just because it's easier to store only a 1 byte integer of metadata seems a bit odd.
In both cases, it would seem to be a subtlety that probably depends on the DB implementation. The practice of using 255 seems pretty widespread, so someone somewhere must have argued a good case for it in the beginning, can anyone remember what that case was/is? Programmers won't adopt any new practice without a reason, and this must have been new once.
With a maximum length of 255 characters, the DBMS can choose to use a single byte to indicate the length of the data in the field. If the limit were 256 or greater, two bytes would be needed.
A value of length zero is certainly valid for varchar data (unless constrained otherwise). Most systems treat such an empty string as distinct from NULL, but some systems (notably Oracle) treat an empty string identically to NULL. For systems where an empty string is not NULL, an additional bit somewhere in the row would be needed to indicate whether the value should be considered NULL or not.
As you note, this is a historical optimisation and is probably not relevant to most systems today.
255 was the varchar limit in mySQL4 and earlier.
Also 255 chars + Null terminator = 256
Or 1 byte length descriptor gives a possible range 0-255 chars
255 is the largest numerical value that can be stored in a single-byte unsigned integer (assuming 8-bit bytes) - hence, applications which store the length of a string for some purpose would prefer 255 over 256 because it means they only have to allocate 1 byte for the "size" variable.
From MySQL Manual:
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
Understand and make choice.
255 is the maximum value of a 8 bit integer : 11111111 = 255.
Are there other magnitudes that provide performance benefits? For example is varchar(512) less performant than varchar(511) or varchar(510)?
Recollected the fundamentals of the bits/bytes storage, it requires one byte to store integers below 256 and two bytes for any integer between 256 and 65536.
Hence, it requires same space (two bytes) to store 511 or 512 or for that matter 65535....
Thus it is clear that the this argument mentioned in the discussion above is N/A for varchar(512) or varchar(511).
A maximum length of 255 allows the database engine to use only 1 byte to store the length of each field. You are correct that 1 byte of space allows you to store 2^8=256 distinct values for the length of the string.
But if you allow the field to store zero-length text strings, you need to be able to store zero in the length. So you can allow 256 distinct length values, starting at zero: 0-255.
It used to be that all strings required a NUL terminator, or "backslash-zero". Updated databases don't have that. It was "255 characters of text" with a "\0" added automatically at the end so the system knew where the string ended. If you said VARCHAR(256), it would end up being 257 and then you'd be in the next register for one character. Wasteful. That's why everything was VARCHAR(255) and VARCHAR(31). Out of habit the 255 seems to have stuck around but the 31's became 32's and the 511's became 512's. That part is weird. It's hard to make myself write VARCHAR(256).
Often varchars are implemented as pascal strings: holding the actual length in the byte #0. The length was therefore bound to 255. (Value of a byte varies from 0 to 255.)
8 bits unsigned = 256 bytes
255 characters + byte 0 for length
I think this might answer your question. Looks like it was the max limit of varchar in earlier systems. I took it off another stackoverflow question.
It's hard to know what the longest postal address is, of course, which is why many people choose a long VARCHAR that is certainly longer than any address. And 255 is customary because it may have been the maximum length of a VARCHAR in some databases in the dawn of time (as well as PostgreSQL until more recently).
Are there disadvantages to using a generic varchar(255) for all text-based fields?
Data is saved in memory in binary system and 0 and 1 are binary digits. Largest binary number that can fit in 1 byte (8-bits) is 11111111 which converts to decimal 255.
I've got some numbers that is now larger than INT can handle.
This is a bit embarrassing, but I honestly don't know exactly what the BIGINT(8) means. Is the 8 in that the max bit value or the max length?
So BIGINT(1) can only be one digit? Or is BIGINT(1) something else? I think TINYINT(1) max is 127, how does that work out?
What is the biggest I can make BIGINT?
What is the largest number I can store in MySQL as an integer?
The number represents how it's displayed - it doesn't affect how the data is stored.
From the manual:
Another extension is supported by MySQL for optionally specifying the display width of integer data types in parentheses following the base keyword for the type (for example, INT(4)). This optional display width may be used by applications to display integer values having a width less than the width specified for the column by left-padding them with spaces. (That is, this width is present in the metadata returned with result sets. Whether it is used or not is up to the application.)
The display width does not constrain the range of values that can be stored in the column, nor the number of digits that are displayed for values having a width exceeding that specified for the column. For example, a column specified as SMALLINT(3) has the usual SMALLINT range of -32768 to 32767, and values outside the range allowed by three characters are displayed using more than three characters.
A BIGINT is always 8 bytes and can store -9223372036854775808 to 9223372036854775807 (signed) or 0 to 18446744073709551615 (unsigned).
You answer is in this overview. A BIGINT is indeed 8 bytes. A TinyInt only 1.
Btw, I don't consider the range from -9223372036854775808 to 9223372036854775807 very embarrassing, it's +/-2^63 :).
The number merely defines the width of the number when being displayed. Have a look at the mysql manual for sizes of numeric types.
The number between brackets is the display width and is unrelated to the range of numbers the datatype can actually store.
In MySQL the BIGINT numeric data type requires 8 bytes for storage and has the following ranges:
The signed range is -9223372036854775808 to 9223372036854775807 (i.e. from -263 to 263-1).
The unsigned range is 0 to 18446744073709551615 (i.e. from 0 to 264-1).
You can find more information here: MySQL Numeric Data Types.
To store a 128-bit integer, you can use a BINARY(16).
For 252:
SELECT RIGHT(CONCAT(REPEAT("\0",16),UNHEX(CONV(252,10,16))),16);
or equivalently:
SELECT UNHEX(RIGHT(CONCAT(REPEAT("0",32),HEX(252)),32));
(but MySQL integer calculations are 64-bit)
See also the recent IPv6 functions INET6_ATON() and INET6_NTOA(),
http://dev.mysql.com/doc/refman/5.6/en/miscellaneous-functions.html#function_inet6-aton
and how they use a VARBINARY(16).
I want to know in sql,how fixed-length data type take places length in memory?I know is that for varchar,if we specify length is (20),and if user input length is 15,it takes 20 by setting space.for varchar2,if we specify length is (20),and if user input is 15,it only take 15 length in memory.So how about fixed-length data type take place?I searched in Google,but I did not find explanation with example.Please explain me with example.Thanks in advance.
A fixed length data field always consumes its full size.
In the old days (FORTRAN), it was padded at the end with space characters. Modern databases might do that too, but either implicitly trim trailing blanks off or the query might have to do it explicitly.
Variable length fields are a relative newcomer to databases, probably in the 1970s or 1980s they made widespread appearances.
It is considerably easier to manage fixed length record offsets and sizes rather than compute the offset of each data item in a record which has variable length fields. Furthermore, a fixed length data record is easily addressed in a data file by computing the byte offset of its beginning by multiplying the record size times the record number (and adding the length of whatever fixed header data is at the beginning of file).