SQL SELECT speed int vs varchar

Question

I'm in the process of creating a table and it made me wonder.

If I store, say cars that has a make (fx BMW, Audi ect.), will it make any difference on the query speed if I store the make as an int or varchar.

So is

SELECT * FROM table WHERE make = 5 AND ...;

Faster/slower than

SELECT * FROM table WHERE make = 'audi' AND ...;

or will the speed be more or less the same?

Solution 1

Int comparisons are faster than varchar comparisons, for the simple fact that ints take up much less space than varchars.

This holds true both for unindexed and indexed access. The fastest way to go is an indexed int column.

---

As I see you've tagged the question postgreql, you might be interested in the space usage of different date types:

• int fields occupy between 2 and 8 bytes, with 4 being usually more than enough ( -2147483648 to [phone] )
• character types occupy 4 bytes plus the actual strings.

Solution 2

Some rough benchmarks:

4 million records in Postgres 9.x

Table A = base table with some columns
Table B = Table A + extra column id of type bigint with random numbers
Table C = Table A + extra column id of type text with random 16-char ASCII strings

Results on 8GB RAM, i7, SSD laptop:

Size on disk:                A=261MB        B=292MB        C=322MB
Non-indexed by id: select count(*), select by id: 450ms same on all tables
Insert* one row per TX:       B=9ms/record        C=9ms/record
Bulk insert* in single TX:    B=140usec/record    C=180usec/record
Indexed by id, select by id:  B=about 200us       C=about 200us

* inserts to the table already containing 4M records

so it looks like for this setup, as long as your indexes fit in RAM, bigint vs 16-char text makes no difference in speed.

Solution 3

It will be a bit faster using an int instead of a varchar. More important for speed is to have an index on the field that the query can use to find the records.

There is another reason to use an int, and that is to normalise the database. Instead of having the text 'Mercedes-Benz' stored thousands of times in the table, you should store it's id and have the brand name stored once in a separate table.

Solution 4

Breaking down to the actual performance of string comparison versus non-floats, in this case any size unsigned and signed does not matter. Size is actually the true difference in performance. Be it 1byte+(up to 126bytes) versus 1,2,4 or 8 byte comparison... obviously non-float are smaller than strings and floats, and thus more CPU friendly in assembly.

String to string comparison in all languages is slower than something that can be compared in 1 instruction by the CPU. Even comparing 8 byte (64bit) on a 32bit CPU is still faster than a VARCHAR(2) or larger. * Again, look at the produced assembly (even by hand) it takes more instructions to compare char by char than 1 to 8 byte CPU numeric.

Now, how much faster? depends also upon the volume of data. If you are simply comparing 5 to 'audi' - and that is all your DB has, the resulting difference is so minimal you would never see it. Depending upon CPU, implementation (client/server, web/script, etc) you probably will not see it until you hit few hundred comparisons on the DB server (maybe even a couple thousand comparisons before it is noticeable).

• To void the incorrect dispute about hash comparisons. Most hashing algorithms themselves are slow, so you do not benefit from things like CRC64 and smaller. For over 12 years I developed search algorithms for multi-county search engines and 7 years for the credit bureaus. Anything you can keep in numeric the faster... for example phone numbers, zip codes, even currency * 1000 (storage) currency div 1000 (retrieval) is faster than DECIMAL for comparisons.

Ozz

Solution 5

Index or not, int is a lot faster (the longer the varchar, the slower it gets).

Another reason: index on varchar field will be much larger than on int. For larger tables it may mean hundreds of megabytes (and thousands of pages). That makes the performance much worse as reading the index alone requires many disk reads.

View more solutions

Comments

[googletorp]

Interesting, How will the speed difference be between ENUM and int?

[Robert Munteanu]

Does PostgresSQL have an enum data type? I though it was MySQL specific.

[googletorp]

Postgres has ENUM, but I don't think it's implemented quite the same way as MySQL. postgresql.org/docs/current/static/datatype-enum.html

[Magnus Hagander]

You are referring to pg 7.4. In modern versions, they take up 1byte+length if you have <126 bytes. Also note that the reason strings are much slower is often that collation-sensitive comparison is hugely expensive - not that the string takes more space. But the end result is the same, of course.

[Magnus Hagander]

Performance wise, ENUM should perform more or less the same as int in the search field, but as varchar in the target list (because it has to transfer the whole string to the client for matched rows, not just the int)

[Robert Munteanu]

@Magnus - thanks for the heads-up. Feel free to edit my answer as I see you have enough rep points.

[Andris]

Could you explain more? Do you mean instead of Mercedes-Benz to store thousands of times id 1. For example table car_brands, columns Brands and Id. Row Mercedes-Benz and 1. And in main table column Brands and value 1. And when SELECT, then at first get Id from table car_brands and then SELECT Something FROM main_table WHERE Brands = (SELECT Id FROM car_brands WHERE Brands = Mercedes-Benz). Or some other approach?

[Guffa]

@user2118559: Yes, that is how you would store it. To get the data you would generally use a join rather than a subquery: select something from main_table c inner join car_brands b on b.Id = c.Brands where b.Brands = 'Mercedes-Benz'.

[Guffa]

Why the downvote? If you don't explain what it is that you think is wrong, it can't improve the answer.

[MrMesees]

"not that the string takes more space"... strings of characters above minimal sizes take up a heck of a lot more space than even high-precision numbers, because a number (singular) has a fixed unit, strings are always aggregate types. 8 bytes for a 64-bit number 4 bytes per-character in a string, including either a length byte or struct; or another terminator character for incredibly naive implementations...

[Wilt]

Here an interesting read on why NOT to use enum in MySQL (just to add some fuel to the fire :D )

[AiRiFiEd]

@RobertMunteanu Hey Robert, apologies I know this is an old post but can I kindly check...on the following: in order to query integers, i have to link each string column to another table (relationship). however, that means more joining operations are required for each query. How do i determine if this trade-off is worth it? Thank you!

[lulalala]

For example of 5 millions records of "audi", wouldn't the index only hold only one copy of string of "audi" and 5 millions integers of primary_key? Would the size difference really be that large, be it vchar or integer?

[Chibueze Opata]

Very interesting. How come the difference is negligible?

[Marcin Wojnarski]

"Int comparisons are faster than varchar comparisons, for the simple fact that ints take up much less space than varchars" - this is NOT true in general. Depending on the DBMS you use and the exact data types and strings you want to insert, it may turn out that your (say) 8-byte ints are longer than ascii varchars holding some textual IDs of avg length 3-4 chars. So, this answer - being imprecise and lacking any specific context or experimental results - doesn't really answer the question. Everyone knows that varchars are allowed to take much more space than ints, but they do NOT have to.

[Awais fiaz]

You are right lulalala but for a column which is going to contain random strings the answer is fair enough.

[Brettins]

Can you back up your claim here about indexed access? Every benchmark I've seen posted online says that varchar vs int are identical for indexed access, and you are posting no data nor reference to back up your claim. stackoverflow.com/a/48583244/834393

[rulhaniam]

One simple thing that suggest search would be faster with integers is due to the comparison between 2 integer vs 2 string (varchar). the compare operation is constant .i.e. O(1) in case of integers but with strings its not constant (but depends on the length of the string).

[Nearoo]

@ChibuezeOpata Before data can be compared, it has to be loaded into caches closer to the CPU than RAM. The overall comparison time is likely completely dominated by that loading. i7 L1 & L2 caches are 64 bytes in size, and you can likely load 64 succing bytes from RAM at once. Hence, as long as your data is smaller than 32 bytes (compare 32 to bytes to each other), the only difference is the cycles the CPU need for comparison, which is negligible. Just a guess, the actual answer is probably super complex.

[Nearoo]

Computers are not that simple to deduce more memory => less performance. Indexed columns are highly processed datastructures whose size might not be a function of the width of the columns: When columns are small, indexes are alrger than the data in the columns to increase performance, and when they're large, they're smaller to increase performance. Further, data loading is completely pararell up to a certain degree. This answer should not be accepted.