postgresql - postgres 10 对 varchars 使用什么哈希算法？

Question

如果我在 varchar 上应用散列索引，postgres 10 将使用什么算法来散列该值？会是MD5吗？杂音3？ FNV-1？我无法在任何地方找到这个记录。

Answer 1

您可以使用此查询找到正确的函数:

SELECT DISTINCT p.amproc
FROM pg_amproc p
   JOIN pg_opfamily f ON p.amprocfamily = f.oid
   JOIN pg_am a ON f.opfmethod = a.oid
WHERE a.amname = 'hash'
  AND p.amproclefttype = 'text'::regtype;

函数是hashtext，它在内部调用backend/access/hash/hashfunc.c的hash_any。

这个函数的注释告诉你更多:

/*
 * This hash function was written by Bob Jenkins
 * (bob_jenkins@burtleburtle.net), and superficially adapted
 * for PostgreSQL by Neil Conway. For more information on this
 * hash function, see http://burtleburtle.net/bob/hash/doobs.html,
 * or Bob's article in Dr. Dobb's Journal, Sept. 1997.
 *
 * In the current code, we have adopted Bob's 2006 update of his hash
 * function to fetch the data a word at a time when it is suitably aligned.
 * This makes for a useful speedup, at the cost of having to maintain
 * four code paths (aligned vs unaligned, and little-endian vs big-endian).
 * It also uses two separate mixing functions mix() and final(), instead
 * of a slower multi-purpose function.
 */