Is the member name of hashctl inappropriate？

The definition of hashctl is shown below

typedef struct HASHCTL
{
long num_partitions; /* # partitions (must be power of 2) */
long ssize; /* segment size */
long dsize; /* (initial) directory size */
long max_dsize; /* limit to dsize if dir
size is limited */
long ffactor; /* fill factor */
Size keysize; /* hash key length in bytes */
Size entrysize; /* total user element size
in bytes */
HashValueFunc hash; /* hash function */
HashCompareFunc match; /* key comparison function */
HashCopyFunc keycopy; /* key copying function */
HashAllocFunc alloc; /* memory allocator */
MemoryContext hcxt; /* memory context to use
for allocations */
HASHHDR *hctl; /* location of header in
shared mem */
} HASHCTL;

/*
* Key copying functions must have this signature. The return value is not
* 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);

According to the description, the keycopy function only copies the key, but
in reality it copies the entire entry, i.e., the key and the value, is the
name wrong? This may make the developer pass in an inappropriate keycopy
parameter when creating the htab.

Thanks

ywgrit <yw987194828(at)gmail(dot)com> writes:
> According to the description, the keycopy function only copies the key, but
> in reality it copies the entire entry, i.e., the key and the value,

On what grounds do you claim that? dynahash.c only ever passes "keysize"
as the size parameter.

regards, tom lane

You are right, I came to this erroneous conclusion based on the following
mishandled experiment: My test program is shown below.

typedef struct ColumnIdentifier
{
Oid relid;
AttrNumber resno;
} ColumnIdentifier;

typedef struct ColumnType
{
ColumnIdentifier colId;
Oid vartype;
} ColumnType;

int
ColumnIdentifier_compare(const void *key1, const void *key2, Size keysize)
{
const ColumnIdentifier *colId_key1 = (const ColumnIdentifier *) key1;
const ColumnIdentifier *colId_key2 = (const ColumnIdentifier *) key2;

return colId_key1->relid == colId_key2->relid && colId_key1->resno ==
colId_key2->resno ? 0 : 1;
}

void *
ColumnIdentifier_copy(void *dest, const void *src, Size keysize)
{
ColumnIdentifier *colId_dest = (ColumnIdentifier *) dest;
ColumnIdentifier *colId_src = (ColumnIdentifier *) src;

colId_dest->relid = colId_src->relid;
colId_dest->resno = colId_src->resno;

return NULL; /* not used */
}

HASHCTL hashctl;
hashctl.hash = tag_hash;
hashctl.match = ColumnIdentifier_compare;
hashctl.keycopy = ColumnIdentifier_copy;
hashctl.keysize = sizeof(ColumnIdentifier);
hashctl.entrysize = sizeof(ColumnType);
HTAB *htab = hash_create("type of column",
512 /* nelem */,
&hashctl,
HASH_ELEM | HASH_FUNCTION |
HASH_COMPARE | HASH_KEYCOPY);
ColumnType *entry = NULL;

ColumnIdentifier *colId = (ColumnIdentifier *)
MemoryContextAllocZero(CurrentMemoryContext,
sizeof(ColumnIdentifier));
ColumnType *coltype = (ColumnType *)
MemoryContextAllocZero(CurrentMemoryContext, sizeof(ColumnType));

coltype->colId.relid = colId->relid = 16384;
coltype->colId.resno = colId->resno = 1;
coltype->vartype = INT4OID;

hash_search(htab, coltype, HASH_ENTER, NULL);
entry = hash_search(htab, colId, HASH_FIND, NULL);

Assert(entry->colId.relid == colId->relid);
Assert(entry->colId.resno == colId->resno);
Assert(entry->vartype == INT4OID); // entry->vartype == 0

As shown above, entry->vartype is not assigned when keycopy copies only the
key. I modified ColumnIdentifier_copy as shown below, the keycopy copies
the entire entry.

void *
ColumnIdentifier_copy(void *dest, const void *src, Size keysize)
{
const ColumnType *coltype_src = (const ColumnType *) src;
const ColumnType *coltype_dest = (const ColumnType *) dest;

coltype_dest->colId->relid = coltype_src->colId->relid;
coltype_dest->colId->resno = coltype_src->colId->resno;
coltype_dest->vartype = coltype_src->vartype;

return NULL; /* not used */
}

The result is that entry->vartype is now the same as var->vartype, which
leads me to believe that keycopy "should" copy the entire entry. Before
sending the initial email, I looked at the implementation of
"hash_search_with_hash_value" and found the line
"hashp->keycopy(ELEMENTKEY(currBucket), keyPtr, keysize)", which made me
wonder how data field is copied into the HTAB?

But at the time I ignored a note above: "Caller is expected to fill the
data field on return". Now I know that the data field needs to be filled
manually, so it was my misuse. Thanks for the correction!

Thanks

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 于2023年9月13日周三 09:36写道：

> ywgrit <yw987194828(at)gmail(dot)com> writes:
> > According to the description, the keycopy function only copies the key,
> but
> > in reality it copies the entire entry, i.e., the key and the value,
>
> On what grounds do you claim that? dynahash.c only ever passes "keysize"
> as the size parameter.
>
> regards, tom lane
>