Re: clarify equalTupleDescs()

On Mon, Feb 12, 2024 at 7:47 PM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
>
> In principle, hashRowType() could process all the fields that
> equalRowTypes() does. But since it's only a hash function, it doesn't
> have to be perfect. (This is also the case for the current
> hashTupleDesc().) I'm not sure where the best tradeoff is.

That's where my confusion comes from.
hashRowType is used in record_type_typmod_hash.
record_type_typmod_hash is within assign_record_type_typmod.

in assign_record_type_typmod:
------------------------------------------------
if (RecordCacheHash == NULL)
{
/* First time through: initialize the hash table */
HASHCTL ctl;
ctl.keysize = sizeof(TupleDesc); /* just the pointer */
ctl.entrysize = sizeof(RecordCacheEntry);
ctl.hash = record_type_typmod_hash;
ctl.match = record_type_typmod_compare;
RecordCacheHash = hash_create("Record information cache", 64,
&ctl,
HASH_ELEM | HASH_FUNCTION | HASH_COMPARE);
/* Also make sure CacheMemoryContext exists */
if (!CacheMemoryContext)
CreateCacheMemoryContext();
}
/*
* Find a hashtable entry for this tuple descriptor. We don't use
* HASH_ENTER yet, because if it's missing, we need to make sure that all
* the allocations succeed before we create the new entry.
*/
recentry = (RecordCacheEntry *) hash_search(RecordCacheHash,
&tupDesc,
HASH_FIND, &found);
------------------------------------------------
based on the comments in hash_create. The above hash_search function
would first use
record_type_typmod_hash to find out candidate entries in a hash table
then use record_type_typmod_compare to compare the given tupDesc with
candidate entries.

Is this how the hash_search in assign_record_type_typmod works?

equalRowTypes processed more fields than hashRowType,
hashRowType comments mentioned equalRowTypes,
maybe we should have some comments in hashRowType explaining why only
hashing natts, tdtypeid, atttypid will be fine.