PostgreSQL: Difference between text and varchar (character varying)
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.
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 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,
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
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
UPDATING BENCHMARKS FOR 2016 (pg9.5+)
And using "pure SQL" benchmarks (without any external script)
use any string_generator with UTF8
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
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 tham standard deviation).
varchar(x)because sometimes it is not a standard, e.g. in
express limits (with same
varcharperformance!) by with
CHECKclause in the
With a negligible loss of performance in INSERT/UPDATE you can also to control ranges and string structure
CHECK(char_length(x)>5 AND char_length(x)<=20 AND x LIKE 'Hello%')
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
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.
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
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
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
true, false, true and not
true, true, true as you might expect.