On 5/18/19 5:39 PM, Peter J. Holzer wrote:
On 2019-05-18 17:14:59 -0500, Ron wrote:On 5/18/19 3:49 PM, Peter J. Holzer wrote: On 2019-05-18 15:19:22 -0500, Ron wrote: On 5/18/19 2:27 PM, Peter J. Holzer wrote: On 2019-05-18 10:49:53 -0700, David G. Johnston wrote: You don’t perform math on a hash That's not generally true. Hashes are used for further computation for example in hash tables or in cryptography. How is it "using math" to use a hash key in a hash lookup table? hash modulo table size. I've seen that used when the tablespace is pre-allocated, and you hash modulo the tablespace page number. (Yes, performance tanks when you start filling up pages.) How do you hash on the (ever growing) table size?The hash function returns a number in a range much larger than the possible number of buckets. 64 bits is a good choice today. To determine the bucket you need to reduce this number to something in the range [0, nr_buckets). This is where modulo comes in: i = h % nr_buckets If the the table fills up, you increase nr_buckets, reallocate and rehash all entries.
Ouch. Response time on a big table would take a serious hit if that rehash happened in the middle of the day on a big OLTP system. Even worse if it were a 24x365 system, because you couldn't schedule an enlargement/rehash during a down period.
-- Angular momentum makes the world go 'round.
