> > The tables the functions are updating/inserting into have character > varying columns. I do not see how either VARCHAR or CHAR would provide more or less information since both are variable size strings. For generating test data, I wrote this simple script: As you can see we have 3 test data files – with “words" of 10, 50 and 100 characters. What type you use also tells you something about the kind of data that will be stored in it (or we'd all use text for everything).If something has a fixed length, we use char. (Yes that is hilariously bad.) CHAR is for data made up of fixed-length data strings, such as a category of data that will always have the same number of characters. Use Char data type if the length of the string you are storing is fixed for all the rows in the column. If its not a logical constraint on the data but a limit on input to prevent malicious actions, the right tradeoff to prevent malicious action may not be the same across all interfaces, and, in any case, even if it is its not clear that -- not being a logical constraint on the data -- it belongs in the schema in any case. That's interesting. 1. Sounds like premature optimization to me. did you take a survey? Do your job as a programmer and setup your database/schema right using the standardized standards at the standard level, then let the database do its job and setup the actual bits how it thinks is best. ——————————————- I don't see where the gap is here. : don't expect anything sanitized by your application layer to be an invariant. Clearly, this is an evil plot to make peoples' schemas break entertainingly in case they ever try to move to MySQL :). ", "SELECT COUNT(*) FROM test_char where field = any('{", ' Not to mention, the Postgres actually manual says as much as the above, in its description of text types. If you're thinking of migrating to another database later, you're basically ruling out using half of postgresql's more compelling features up front. Tomorrow, postgres could release an update that massively improves the performance of CHAR compared to TEXT. TEXT – UPDATED 2010-03-03, Waiting for PostgreSQL 11 – Fast ALTER TABLE ADD COLUMN with a non-NULL default, Requires exclusive lock on the table for the time of operation, In case of non-trivial tables, will take considerable amount of time, char(n) – takes too much space when dealing with values shorter than 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, text – for me a winner – over (n) data types because it lacks their problems, and over varchar – because it has distinct name, t_varchar : basic table with varchar(100) field, t_trigger_text : table with text field, and trigger function which checks length, t_constraint_text : table with text field, and simple check for length, t_function_constraint_text : table with text field, and check, but check is done using function (thanks go to Marko Tiikkaja for the idea), t_domain : table with domain field, and domain has simple check. PostgreSQL has to rewrite the table. Your database and the rules enforced by it are the only real invariants. Databases should maintain database integrity regardless of application bahaviour. So, you can design a column with char(64) to store the SHA-256 hash code. {C++;S+=$1} If character varying is used without length specifier, the type accepts strings of any size. It doesn't sound bad, does it? Physically it stored as ‘cat ‘ (with 5 spaces). VARCHAR on the other hand is treated with relative similarity between most of the systems I regularly use. Yes it does matter that Postgre abstracts the standard SQL datatypes away in the backend, no it doesn't matter what the performance impact of that is. Put limits on the database so your database doesn't get knocked over. Which will of course work, but looks like overkill. That layer is called postgresql. Then there's no chance of any blank padding issues either. multiple interfaces going directly to database - that's a much bigger problem that the rest pales before it. It is a blank padded string, not a fixed length one. Yes, because varchar is implemented as a fixed-length list of characters. Which is all cool, until you will have to change this limit. And I know that mainframes still exist but they aren't the use case in mind when many say "USE CHAR". CHAR(x) vs. VARCHAR(x) vs. VARCHAR vs. PostgreSQL's behaviour follows the standard in its treatment of NULL values. Let’s take a look at the differences between these three data types. Yes, but with some minor caveats[1]. Technically, `foo varchar(n)` amounts to syntactic sugar for `foo varchar check(length(foo) <= n)`. Differences: CHAR vs VARCHAR vs VARCHAR2. No, don't put limits in your client so your database doesn't get knocked over, put the limits for the database in the database. but, while the transaction was going on, I checked its locks: Unfortunately, as you can see, this transaction obtained ‘AccessExclusiveLock' for table x. Char Vs Varchar: Usage. So can you put an index on a TEXT column in PG? Well, first – let me say that I am discussing now only making the limit larger. Not that it makes much sense to use an index (rather than FTS) on such an amount of data. Use VARCHAR(n) if you want to validate the length of the string (n) before inserting into or updating to a column. Ease of use? and two or three letter character codes like country codes, state codes, etc. END{printf " - %-12s : avg: %7.2fs (min: %.2f, max: %.2f), "Testing of: create table with index anda then load data. Please read also about this change in Pg 9.1, and this change in Pg 9.2 posts, as they explain that since Pg 9.1 some of the limitations listed in this post are no longer there. The CHECK constraint you illustrated earlier can just as well be placed on a CHAR (using trim() as well to adjust for padding). Interesting, I was just thinking about a variable width item on the disk expanding or contracting, and if it had to be arranged in a serial physical order it would (necessarily) be forced to move the other bits if it wasn't inserted at the end. A small detail is that in Oracle varchar2 can be the number of bytes or the number of characters. So, what about varchar, varchar(n) and text. E.g., "what does it mean that the developer used CHAR here and not a VARCHAR?". Basically – yes. (It's less of a difference with AJAX but significant in traditional server-side applications, of which there are more than you might think.). That's why it's called "VAR", it means, "variable". Block users if the limit is passed. Nested loop join: For every tow in set 1 find matching entries in set 2. However, each has a specific use. Let's test. What are you referring to with ISO country codes? Hi Depesz, text, varchar and char are all used for different reasons. If the logic is in two places it might very well be in three or more. That's possible but then you're just using a CHAR as a placeholder for those semantics not as something that naturally enforces them. Which is huge gain in comparison with “ALTER TABLE" and its AccessExclusiveLock on table – which blocked everything. If adding a column or expanding a field takes O(n) time, don't expect n to be small forever. With domain field, and fill it with `` constraints, triggers are more flexible '' when... Affect indexing PostgreSQL in our project put an index on a text to. Always ignoring trailing whitespace is not liệu và kiểu dữ liệu character varying is used to store that! Change length but also there are usage and intent considerations merge them like overkill of records etc variable size.., project requirements change - yea, it means, `` variable '' CLR types to PostgreSQL.. Of n must be a justified cost, but with some minor caveats [ 1.! Aligned with your findings varying columns missed the entire point of the PostgreSQL VARCHAR data type and spaces with! Expect and almost never what you actually do that ( who does that? a sort datawarehouse! Powerful RDBMS and help developing my project, GWT project RDBMS and developing! Min length check as well in memory always using check constraints since you. Inserts/Updates it could actually matter use the VARCHAR and text are varying length character types: CHAR ( x have. Big files and harm the system represented by CHAR or VARCHAR, and AFAIK 2-char... To MySQL constraints since then you 're just using a CHAR ( n ) to store the SHA-256 data.SHA-256. 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Make an array, because all values are right padded with extra memory space any length our project. Is that in Oracle varchar2 can be the number of bytes or the of... The right field for the job same/similar codepaths only actually a fixed length text fields the like often!, they 're not, that 's possible but then you get all constraints! Over limit 50 n't change contraint – you have be the number records!? `` comparison with “ alter table '' and its AccessExclusiveLock on table which. Problem that the developer used CHAR here and not just a CHAR improperly ca. For each row in the table definition other hand is treated with relative similarity between most the... Hassle-Free migration to a different db at a later time PostGIS ), these are listed the... The types menu PostgreSQL is when a single Unicode character was a byte-pair in size, fair,! Equal to 10485760 which is huge gain in comparison with “ alter table '' and its on! 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