![]() It is also generated from the md5() function, which wants text as input value. There is a hexadecimal form of a 16 bytes number where each byte is encoded using two hexadecimal digits (from 0 to 9 and from a to f). UUID is a 128-bit number, which also means 16 bytes. Generating UUID From Random Bits in PostgreSQL This doesn’t mean you can throw there 128 random bits. There is also UUID Version 4, which is described as random. Of course, you can change the time, you can change the MACs - just be careful. The uniqueness between the machines is guaranteed by the uniqueness of the MAC addresses. The uniqueness of the generated UUIDs on one machine is guaranteed by the time part. This is the version you want to use when generating values on different machines. However, one variant contains MAC address of the computer generating it, some time based values, and some random data. There is a couple of variants of the UUID, in reality none of them is “universally unique”. Unfortunately programmers too often think: “oh, it’s globally unique… so let’s use some random bits there”. “A universally unique identifier ( UUID) is a 128-bit number used to identify information in computer systems.” It also looks nicer when you enlarge the page (as it’s an SVG file): Generating Random UUID in PostgreSQL To have an efficient generation of realistic strings I’d make a PostgreSQL module written in C. I know, the md5_random_string is not the best thing for generating realistic texts. The below times are also for 1k function calls. The slowest function is 4 times faster now. I made benchmarks of the functions again to check how they work on the current PostgreSQL (v12) and on modern hardware. One drawback of this implementation is that it doesn’t use all the letters, it uses text generated from the md5() function, so the possible characters are: 0-9A-F. There is a detailed discussion of the used formula and why a commonly used one is wrong, so here I will just show the code.ĬREATE FUNCTION random_text( INTEGER) RETURNS TEXT LANGUAGE SQL AS $$ select upper( substring( ( SELECT string_agg(md5(random():: TEXT), '') FROM generate_series( 1, CEIL( $ 1 / 32.):: integer) ), 1, $ 1) ) $$ The standard random() function returns a number from a range of (which includes the min and the max values). Generating Random Integer In a RangeĪnother useful function is generating a random integer in a given range. 7111753978720401Īs you can see – after setting the internal generator state to the same value, the sequence of the generated random numbers is the same. $ select random(), random(), random() random | random | random -+-+-Ġ. This way you will get the same list of numbers every time. If you want to generate the same list of random numbers (fine, numbers which look like they are random) – you should set the initial state of the generator to the same value, right before generating the first number. Using the setseed() function is important for reproducible data generation. This kind of random number generation should be fine for most of the use cases. The initial state can be set by the setseed() function. The state is modified every time a number is generated and the new state depends on the previous one. It means that the next generated number depends on the generator’s internal state. The algorithm used by the random() function generates pseudo-random values. ![]() setseed() – sets the internal value for the generator.random() – returns a random value with uniform distribution from the range [0.0, 1.0) (includes 0.0, but no 1.0).PostgreSQL has two basic functions for generating random data: Here I’m trying to describe common problems I met when using PostgreSQL and how they can be solved. Sometimes the distribution of the random numbers is not what we wanted. Sometimes we get a little bit unpredictable results. Dealing with randomness in PostgreSQL can be tricky.
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