Recently I had to use GUID (Global Unique Identifier) as primary key in order to ensure cross-server uniqueness. In many cases surrogate key (some sort of concatenation between server id and intra-server identity) would give a better performance but in my case due to application requirements surrogate key wasn't an option.

Next step is partitioning by GUID column. Say, you want 3 partitions. Ho do you build partition function? Sure, you can perform a calculation and find the exact 2 values that will divide all the GUID range to 3 equal parts. I needed at least 20 partitions, so I used an approximation. If you write down long hexadecimal number, you'll receive something like A09F76... I decided that partitioning by first 2 tabs is enough. Example for decimal: let's divide values from 1 to 10000 to 3 partitions - what would be the ranges? The exact answer is: 1 - 3333, 3334 - 6666, 6667-10000. Using approximation by 2 first digits will give the following ranges: 1 - 3300, 3301 - 6600, 6601 - 10000. 2 orders of magnitude (first 2 digits) are good enough approximation. In hexadecimal first 2 digits (00 - FF) represent 0-255 in decimal. I'm not accustomed to hexadecimal calculations, so first I divide decimal range to 3 parts: 1 - 85, 86 - 170, 171 - 255. Next step is converting it to hexadecimal: 01 - 55, 56 - AA, AB - FF.

OK, done with values, how do I translate them into GUID? For example, 01 in hexadecimal - what is the GUID representation of it (taking into account that those should be first 2 digits - not the last ones). Took me awhile to understand. First I tried "01000000-0000-0000-0000-000000000000" - surprisingly all the rows (more then a million) came to the first partition. OK, another try: "00000001-0000-0000-0000-000000000000". Still the same. The catch is that GUID should be read in a byte groups order from right to left while inside every byte group reading order is from left to right. And every 2 tabs are 1 byte. So the right representation for 01 as first 2 digits in GUID is: "00000000-0000-0000-0000-010000000000".

To finish with, partition function that partitions GUID to 20 range groups: