Rotate instead of shifting hash join batch number.

Our algorithm for choosing batch numbers turned out not to work
effectively for multi-billion key inner relations.  We would use
more hash bits than we have, and effectively concentrate all tuples
into a smaller number of batches than we intended.  While ideally
we should switch to wider hashes, for now, change the algorithm to
one that effectively gives up bits from the bucket number when we
don't have enough bits.  That means we'll finish up with longer
bucket chains than would be ideal, but that's better than having
batches that don't fit in work_mem and can't be divided.

Batch-patch to all supported releases.

Author: Thomas Munro
Reviewed-by: Tom Lane, thanks also to Tomas Vondra, Alvaro Herrera, Andres Freund for testing and discussion
Reported-by: James Coleman
Discussion: https://postgr.es/m/16104-dc11ed911f1ab9df%40postgresql.org

1 files changed, 9 insertions, 0 deletions

diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 5197926696f..d1544834a6b 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -136,4 +136,13 @@ extern int	(*pg_popcount64) (uint64 word);
 /* Count the number of one-bits in a byte array */
 extern uint64 pg_popcount(const char *buf, int bytes);
 
+/*
+ * Rotate the bits of "word" to the right by n bits.
+ */
+static inline uint32
+pg_rotate_right32(uint32 word, int n)
+{
+	return (word >> n) | (word << (sizeof(word) * BITS_PER_BYTE - n));
+}
+
 #endif							/* PG_BITUTILS_H */