What's the difference between the text data type and the character varying (varchar) data types?
According to the documentation
If character varying is used without length specifier, the type accepts strings of any size. The latter is a PostgreSQL extension.
and
In addition, PostgreSQL provides the text type, which stores strings of any length. Although the type text is not in the SQL standard, several other SQL database management systems have it as well.
So what's the difference?
There is no difference, under the hood it's all varlena (variable length array).
Check this article from Depesz: http://www.depesz.com/index.php/2010/03/02/charx-vs-varcharx-vs-varchar-vs-text/
A couple of highlights:
To sum it all up:
- char(n) – takes too much space when dealing with values shorter than
n(pads them ton), and can lead to subtle errors because of adding trailing spaces, plus it is problematic to change the limit- varchar(n) – it's problematic to change the limit in live environment (requires exclusive lock while altering table)
- varchar – just like text
- text – for me a winner – over (n) data types because it lacks their problems, and over varchar – because it has distinct name
The article does detailed testing to show that the performance of inserts and selects for all 4 data types are similar. It also takes a detailed look at alternate ways on constraining the length when needed. Function based constraints or domains provide the advantage of instant increase of the length constraint, and on the basis that decreasing a string length constraint is rare, depesz concludes that one of them is usually the best choice for a length limit.
text should never, ever be considered "a winner over varchar" out of the box just because it allows me to input strings of any length, but exactly the opposite, you should really think about what kind of data you want to store before allowing your users to input strings of any lenght. And NO, "let the Frontend handle it" is definitely not acceptable and a very bad development practice. Really surprising to see a lot of devs doing this nowadays. –
Fernandez text, and if you need a length limit, use a check constraint or domain." –
Wideopen varchar(n) over text because it allows me to increase the length without the forced rechecking or hackish solutions (e.g. function constraints) provided in OP's article. –
Wideopen As "Character Types" in the documentation points out, varchar(n), char(n), and text are all stored the same way. The only difference is extra cycles are needed to check the length, if one is given, and the extra space and time required if padding is needed for char(n).
However, when you only need to store a single character, there is a slight performance advantage to using the special type "char" (keep the double-quotes — they're part of the type name). You get faster access to the field, and there is no overhead to store the length.
I just made a table of 1,000,000 random "char" chosen from the lower-case alphabet. A query to get a frequency distribution (select count(*), field ... group by field) takes about 650 milliseconds, vs about 760 on the same data using a text field.
"char" is not char?? It is valid in nowadays of PostgreSQL 11+? ... Yes: "The type "char" (note the quotes) is different from char(1) in that it only uses one byte of storage. It is internally used in the system catalogs as a simplistic enumeration type.", guide/datatype-character. –
Tletski (this answer is a Wiki, you can edit - please correct and improve!)
And using "pure SQL" benchmarks (without any external script)
use any string_generator with UTF8
main benchmarks:
2.1. INSERT
2.2. SELECT comparing and counting
CREATE FUNCTION string_generator(int DEFAULT 20,int DEFAULT 10) RETURNS text AS $f$
SELECT array_to_string( array_agg(
substring(md5(random()::text),1,$1)||chr( 9824 + (random()*10)::int )
), ' ' ) as s
FROM generate_series(1, $2) i(x);
$f$ LANGUAGE SQL IMMUTABLE;
Prepare specific test (examples)
DROP TABLE IF EXISTS test;
-- CREATE TABLE test ( f varchar(500));
-- CREATE TABLE test ( f text);
CREATE TABLE test ( f text CHECK(char_length(f)<=500) );
Perform a basic test:
INSERT INTO test
SELECT string_generator(20+(random()*(i%11))::int)
FROM generate_series(1, 99000) t(i);
And other tests,
CREATE INDEX q on test (f);
SELECT count(*) FROM (
SELECT substring(f,1,1) || f FROM test WHERE f<'a0' ORDER BY 1 LIMIT 80000
) t;
... And use EXPLAIN ANALYZE.
UPDATED AGAIN 2018 (pg10)
little edit to add 2018's results and reinforce recommendations.
My results, after average, in many machines and many tests: all the same
(statistically less than standard deviation).
Use text datatype,
avoid old varchar(x) because sometimes it is not a standard, e.g. in CREATE FUNCTION clauses varchar(x)≠varchar(y).
express limits (with same varchar performance!) by with CHECK clause in the CREATE TABLE
e.g. CHECK(char_length(x)<=10).
With a negligible loss of performance in INSERT/UPDATE you can also to control ranges and string structure
e.g. CHECK(char_length(x)>5 AND char_length(x)<=20 AND x LIKE 'Hello%')
"char", that is not char, even in nowadays of PostgreSQL 11+. As the guide/datatype-character says "The type "char" (note the quotes) is different from char(1) in that it only uses one byte of storage. It is internally used in the system catalogs as a simplistic enumeration type.". –
Tletski On PostgreSQL manual
There is no performance difference among these three types, apart from increased storage space when using the blank-padded type, and a few extra CPU cycles to check the length when storing into a length-constrained column. While character(n) has performance advantages in some other database systems, there is no such advantage in PostgreSQL; in fact character(n) is usually the slowest of the three because of its additional storage costs. In most situations text or character varying should be used instead.
I usually use text
References: http://www.postgresql.org/docs/current/static/datatype-character.html
In my opinion, varchar(n) has it's own advantages. Yes, they all use the same underlying type and all that. But, it should be pointed out that indexes in PostgreSQL has its size limit of 2712 bytes per row.
TL;DR:
If you use text type without a constraint and have indexes on these columns, it is very possible that you hit this limit for some of your columns and get error when you try to insert data but with using varchar(n), you can prevent it.
Some more details: The problem here is that PostgreSQL doesn't give any exceptions when creating indexes for text type or varchar(n) where n is greater than 2712. However, it will give error when a record with compressed size of greater than 2712 is tried to be inserted. It means that you can insert 100.000 character of string which is composed by repetitive characters easily because it will be compressed far below 2712 but you may not be able to insert some string with 4000 characters because the compressed size is greater than 2712 bytes. Using varchar(n) where n is not too much greater than 2712, you're safe from these errors.
text and varchar have different implicit type conversions. The biggest impact that I've noticed is handling of trailing spaces. For example ...
select ' '::char = ' '::varchar, ' '::char = ' '::text, ' '::varchar = ' '::text
returns true, false, true and not true, true, true as you might expect.
= operator is not only comparing the stuff, but it also does some conversions to find a common type for the values. It's pretty common behaviour in various languages, and the used conversions also differ between languages. For example in JavaScript you can see that [0 == '0.0', 0 == '0', '0.0' == '0'] -> [true, true, false] –
Flavourful The difference is between tradition and modern.
Traditionally you were required to specify the width of each table column. If you specify too much width, expensive storage space is wasted, but if you specify too little width, some data will not fit. Then you would resize the column, and had to change a lot of connected software, fix introduced bugs, which is all very cumbersome.
Modern systems allow for unlimited string storage with dynamic storage allocation, so the incidental large string would be stored just fine without much waste of storage of small data items.
While a lot of programming languages have adopted a data type of 'string' with unlimited size, like C#, javascript, java, etc, a database like Oracle did not.
Now that PostgreSQL supports 'text', a lot of programmers are still used to VARCHAR(N), and reason like: yes, text is the same as VARCHAR, except that with VARCHAR you MAY add a limit N, so VARCHAR is more flexible.
You might as well reason:
why should we bother using the mouthful "VARCHAR WITHOUT N", now that we can simplify our life with just "TEXT"?
In my recent years with Oracle, I have used CHAR(N) or VARCHAR(N) on very few occasions. Because Oracle does (did?) not have an unlimited string type, I used for most string columns VARCHAR(2000), where 2000 was at some time the maximum for VARCHAR, and in all practical purposes not much different from 'infinite'.
Now that I am working with PostgreSQL, I see TEXT as real progress. No more emphasis on the VAR feature of the CHAR type. No more emphasis on let's use VARCHAR without N. Besides, typing TEXT saves 3 keystrokes compared to VARCHAR.
Younger colleagues would now grow up without even knowing that in the old days there were no unlimited strings. Just like that in most projects they don't have to know about assembly programming.
Update: Azure type String
Apparently, the modern system of Azure SQL has a generic text type named String, like PostgreSQL type Text, but with an unconfigurable limit of just 500 characters. In Azure, type String seems more commonly used than Varchar(N) which has a limit of 4000. Is this progress?
A good explanation from http://www.sqlines.com/postgresql/datatypes/text:
The only difference between TEXT and VARCHAR(n) is that you can limit the maximum length of a VARCHAR column, for example, VARCHAR(255) does not allow inserting a string more than 255 characters long.
Both TEXT and VARCHAR have the upper limit at 1 Gb, and there is no performance difference among them (according to the PostgreSQL documentation).
Somewhat OT: If you're using Rails, the standard formatting of webpages may be different. For data entry forms text boxes are scrollable, but character varying (Rails string) boxes are one-line. Show views are as long as needed.
If you only use TEXT type you can run into issues when using AWS Database Migration Service:
Large objects (LOBs) are used but target LOB columns are not nullable
Due to their unknown and sometimes large size, large objects (LOBs) require more processing and resources than standard objects. To help with tuning migrations of systems that contain LOBs, AWS DMS offers the following options
If you are only sticking to PostgreSQL for everything probably you're fine. But if you are going to interact with your db via ODBC or external tools like DMS you should consider not using TEXT for everything.
I wasted way too much time because of using varchar instead of text for PostgreSQL arrays.
PostgreSQL Array operators do not work with string columns. Refer these links for more details: (https://github.com/rails/rails/issues/13127) and (http://adamsanderson.github.io/railsconf_2013/?full#10).
I have found another annoying difference between them, which gave me a bit of a hard time.
Although VARCHAR (without a size) and TEXT mean more or less the same thing, PostgreSQL still draws a distinction.
The string_agg() function expects either text or bytea data types, and will return the corresponding data type. That doesn’t stop you from using it with the other string data types, such as varchar and its variations.
However, when using it within a user defined function, you can get into strife. For example:
CREATE FUNCTION test(genrename VARCHAR)
RETURNS TABLE (category VARCHAR, items VARCHAR)
LANGUAGE PLPGSQL AS $$ BEGIN
RETURN QUERY
SELECT cat, string_agg(item, '|')
FROM data
GROUP BY cat
END $$;
This will result in an error, since the return type of string_agg is not the same as varchar. Changing the return table to items text fixes this.
In other words, PostgreSQL may treat them as the same, it still maintains a pedantic distinction between varchar and text.
character varying(n), varchar(n) - (Both the same). value will be truncated to n characters without raising an error.
character(n), char(n) - (Both the same). fixed-length and will pad with blanks till the end of the length.
text - Unlimited length.
Example:
Table test:
a character(7)
b varchar(7)
insert "ok " to a
insert "ok " to b
We get the results:
a | (a)char_length | b | (b)char_length
----------+----------------+-------+----------------
"ok "| 7 | "ok" | 2
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