Re: Is the member name of hashctl inappropriate？

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
>