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Commit7149b12

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Improve hash_array() logic for combining hash values.
The new logic is less vulnerable to transpositions.This invalidates the contents of hash indexes built with the oldfunctions; hence, bump catversion.Dean Rasheed
1 parentc58b945 commit7149b12

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2 files changed

+12
-6
lines changed

2 files changed

+12
-6
lines changed

‎src/backend/utils/adt/arrayfuncs.c

Lines changed: 11 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -3533,7 +3533,7 @@ hash_array(PG_FUNCTION_ARGS)
35333533
intndims=ARR_NDIM(array);
35343534
int*dims=ARR_DIMS(array);
35353535
Oidelement_type=ARR_ELEMTYPE(array);
3536-
uint32result=0;
3536+
uint32result=1;
35373537
intnitems;
35383538
TypeCacheEntry*typentry;
35393539
inttyplen;
@@ -3617,11 +3617,17 @@ hash_array(PG_FUNCTION_ARGS)
36173617
}
36183618

36193619
/*
3620-
* Combine hash values of successive elements by rotating the previous
3621-
* value left 1 bit, then XOR'ing in the new element's hash value.
3620+
* Combine hash values of successive elements by multiplying the
3621+
* current value by 31 and adding on the new element's hash value.
3622+
*
3623+
* The result is a sum in which each element's hash value is
3624+
* multiplied by a different power of 31. This is modulo 2^32
3625+
* arithmetic, and the powers of 31 modulo 2^32 form a cyclic group of
3626+
* order 2^27. So for arrays of up to 2^27 elements, each element's
3627+
* hash value is multiplied by a different (odd) number, resulting in
3628+
* a good mixing of all the elements' hash values.
36223629
*/
3623-
result= (result <<1) | (result >>31);
3624-
result ^=elthash;
3630+
result= (result <<5)-result+elthash;
36253631
}
36263632

36273633
/* Avoid leaking memory when handed toasted input. */

‎src/include/catalog/catversion.h

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -53,6 +53,6 @@
5353
*/
5454

5555
/*yyyymmddN */
56-
#defineCATALOG_VERSION_NO201105131
56+
#defineCATALOG_VERSION_NO201105231
5757

5858
#endif

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