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From: Antonin Houska <ah@cybertec.at>
To: pgsql-hackers@postgresql.org
Subject: Comment fix and question about dshash.c
Date: Fri, 26 Oct 2018 13:32:01 +0200
Message-ID: <8726.1540553521@localhost> (raw)

1. The return type of resize() function is void, so I propose part of the
header comment to be removed:

inline patch 1: src/backend/lib/dshash.c (+1/-3 lines) download inline-1.patch
diff --git a/src/backend/lib/dshash.c b/src/backend/lib/dshash.c
index b46f7c4cfd..b2b8fe60e1 100644
--- a/src/backend/lib/dshash.c
+++ b/src/backend/lib/dshash.c
@@ -672,9 +672,7 @@ delete_item(dshash_table *hash_table, dshash_table_item *item)
 
 /*
  * Grow the hash table if necessary to the requested number of buckets.  The
- * requested size must be double some previously observed size.  Returns true
- * if the table was successfully expanded or found to be big enough already
- * (because another backend expanded it).
+ * requested size must be double some previously observed size.
  *
  * Must be called without any partition lock held.
  */


2. Can anyone please explain this macro?

/* Max entries before we need to grow.  Half + quarter = 75% load factor. */
#define MAX_COUNT_PER_PARTITION(hash_table)				\
	(BUCKETS_PER_PARTITION(hash_table->size_log2) / 2 + \
	 BUCKETS_PER_PARTITION(hash_table->size_log2) / 4)

I'm failing to understand why the maximum number of hash table entries in a
partition should be smaller than the number of buckets in that partition.

The fact that MAX_COUNT_PER_PARTITION refers to entries and not buckets is
obvious from this condition in dshash_find_or_insert()

	/* Check if we are getting too full. */
	if (partition->count > MAX_COUNT_PER_PARTITION(hash_table))

-- 
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com

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