Re: Is the member name of hashctl inappropriate？ - Mailing list 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