Re: Is the member name of hashctl inappropriate?
От | ywgrit |
---|---|
Тема | Re: Is the member name of hashctl inappropriate? |
Дата | |
Msg-id | CAPt2h2ZaUTz3m+VdhfaDU_0Q4Zd3CGE_zS058EmuC39e-ncmBg@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Is the member name of hashctl inappropriate? (Tom Lane <tgl@sss.pgh.pa.us>) |
Список | pgsql-hackers |
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
ywgrit <yw987194828@gmail.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
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