Difference between text and varchar (character varying)

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 to n), 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.

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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.

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(this answer is a Wiki, you can edit - please correct and improve!)

UPDATING BENCHMARKS FOR 2016 (pg9.5+)

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.

Results in 2016 and 2018

My results, after average, in many machines and many tests: all the same (statistically less than standard deviation).

Recommendation

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%')

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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

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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.

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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.

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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.

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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).

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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).

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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 using not using TEXT for everything.

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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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