Improve hash_create's API for selecting simple-binary-key hash functions.

Previously, if you wanted anything besides C-string hash keys, you had to
specify a custom hashing function to hash_create().  Nearly all such
callers were specifying tag_hash or oid_hash; which is tedious, and rather
error-prone, since a caller could easily miss the opportunity to optimize
by using hash_uint32 when appropriate.  Replace this with a design whereby
callers using simple binary-data keys just specify HASH_BLOBS and don't
need to mess with specific support functions.  hash_create() itself will
take care of optimizing when the key size is four bytes.

This nets out saving a few hundred bytes of code space, and offers
a measurable performance improvement in tidbitmap.c (which was not
exploiting the opportunity to use hash_uint32 for its 4-byte keys).
There might be some wins elsewhere too, I didn't analyze closely.

In future we could look into offering a similar optimized hashing function
for 8-byte keys.  Under this design that could be done in a centralized
and machine-independent fashion, whereas getting it right for keys of
platform-dependent sizes would've been notationally painful before.

For the moment, the old way still works fine, so as not to break source
code compatibility for loadable modules.  Eventually we might want to
remove tag_hash and friends from the exported API altogether, since there's
no real need for them to be explicitly referenced from outside dynahash.c.

Teodor Sigaev and Tom Lane

1 files changed, 22 insertions, 15 deletions

diff --git a/src/include/utils/hsearch.h b/src/include/utils/hsearch.h
index 77974a193b2..dfdf658ddf0 100644
--- a/src/include/utils/hsearch.h
+++ b/src/include/utils/hsearch.h
@@ -31,7 +31,7 @@ typedef int (*HashCompareFunc) (const void *key1, const void *key2,
 
 /*
  * Key copying functions must have this signature.  The return value is not
- * used.  (The definition is set up to allow memcpy() and strncpy() to be
+ * used.  (The definition is set up to allow memcpy() and strlcpy() to be
  * used directly.)
  */
 typedef void *(*HashCopyFunc) (void *dest, const void *src, Size keysize);
@@ -80,19 +80,20 @@ typedef struct HASHCTL
 } HASHCTL;
 
 /* Flags to indicate which parameters are supplied */
-#define HASH_PARTITION	0x001	/* Hashtable is used w/partitioned locking */
-#define HASH_SEGMENT	0x002	/* Set segment size */
-#define HASH_DIRSIZE	0x004	/* Set directory size (initial and max) */
-#define HASH_FFACTOR	0x008	/* Set fill factor */
-#define HASH_FUNCTION	0x010	/* Set user defined hash function */
-#define HASH_ELEM		0x020	/* Set keysize and entrysize */
-#define HASH_SHARED_MEM 0x040	/* Hashtable is in shared memory */
-#define HASH_ATTACH		0x080	/* Do not initialize hctl */
-#define HASH_ALLOC		0x100	/* Set memory allocator */
-#define HASH_CONTEXT	0x200	/* Set memory allocation context */
-#define HASH_COMPARE	0x400	/* Set user defined comparison function */
-#define HASH_KEYCOPY	0x800	/* Set user defined key-copying function */
-#define HASH_FIXED_SIZE 0x1000	/* Initial size is a hard limit */
+#define HASH_PARTITION	0x0001	/* Hashtable is used w/partitioned locking */
+#define HASH_SEGMENT	0x0002	/* Set segment size */
+#define HASH_DIRSIZE	0x0004	/* Set directory size (initial and max) */
+#define HASH_FFACTOR	0x0008	/* Set fill factor */
+#define HASH_ELEM		0x0010	/* Set keysize and entrysize */
+#define HASH_BLOBS		0x0020	/* Select support functions for binary keys */
+#define HASH_FUNCTION	0x0040	/* Set user defined hash function */
+#define HASH_COMPARE	0x0080	/* Set user defined comparison function */
+#define HASH_KEYCOPY	0x0100	/* Set user defined key-copying function */
+#define HASH_ALLOC		0x0200	/* Set memory allocator */
+#define HASH_CONTEXT	0x0400	/* Set memory allocation context */
+#define HASH_SHARED_MEM 0x0800	/* Hashtable is in shared memory */
+#define HASH_ATTACH		0x1000	/* Do not initialize hctl */
+#define HASH_FIXED_SIZE 0x2000	/* Initial size is a hard limit */
 
 
 /* max_dsize value to indicate expansible directory */
@@ -143,11 +144,17 @@ extern void AtEOSubXact_HashTables(bool isCommit, int nestDepth);
 
 /*
  * prototypes for functions in hashfn.c
+ *
+ * Note: It is deprecated for callers of hash_create to explicitly specify
+ * string_hash, tag_hash, uint32_hash, or oid_hash.  Just set HASH_BLOBS or
+ * not.  Use HASH_FUNCTION only when you want something other than those.
  */
 extern uint32 string_hash(const void *key, Size keysize);
 extern uint32 tag_hash(const void *key, Size keysize);
-extern uint32 oid_hash(const void *key, Size keysize);
+extern uint32 uint32_hash(const void *key, Size keysize);
 extern uint32 bitmap_hash(const void *key, Size keysize);
 extern int	bitmap_match(const void *key1, const void *key2, Size keysize);
 
+#define oid_hash uint32_hash	/* Remove me eventually */
+
 #endif   /* HSEARCH_H */