Re: GiST intarray rd-tree indexes using intbig

> It is documented that intbig utilizes 4096 bit signatures to represent
> the set nodes in the tree. However, I am unable to find any reference
> to a 4kbit signature in the code and am wondering where this is implemented.
_int.h:
/* bigint defines */
#define SIGLENINT 63 /* >122 => key will toast, so very slow!!! */
#define SIGLEN ( sizeof(int)*SIGLENINT )
#define SIGLENBIT (SIGLEN*BITS_PER_BYTE)

63 /*ints*/ * 4 /* byte per int */ * 8 /* bits per byte */ = 2016 bits

From our experience, using power of 2 number of bits causes bad hashing of array.

You can play with value of SIGLENINT:
- less value decreases index's size, but increase number of false drops, so
index will return more values and pgsql will drop it after rechecking of
table's value. That increases table's access
- greater value increase index's size but decrease number of false drops.

So, you can find optimal SIGLENINT value for your sets of data.

> Also, is the leaf comparison also a signature comparison like the
> nodes? Or is this an exact comparison? From my understanding of the

What do you mean: comparison of signatures? RD-Tree doesn't use any comparison
functions like B-Tree does. Here we use distance function. Distance might be
defined in different meaning, but we use Hemming distance
(_intbig_gist.c:hemdistsign) which is number of different bits in signatures.

> code, it doesn’t appear to be an exact comparison. If this is the case,
> how can I access the original intarray that is being referenced by this
> signature?

Index doesn't store original int[] at all. From GiST support fuction there is no
way to get access to table's value :(.

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Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/