Function SysCacheGetAttr

src/backend/utils/cache/syscache.c:1616–1639

* SysCacheGetAttr * * Given a tuple previously fetched by SearchSysCache(), * extract a specific attribute. * * This is equivalent to using heap_getattr() on a tuple fetched * from a non-cached relation. Usually, this is only used for attributes * that could be NULL or variable length; the fixed-size attributes in * a system table are accessed just by mapping the tuple onto the C struct

 * a different cache for the same catalog the tuple was fetched from.
 */
Datum
SysCacheGetAttr(int cacheId, HeapTuple tup,
				AttrNumber attributeNumber,
				bool *isNull)
{
	/*
	 * We just need to get the TupleDesc out of the cache entry, and then we
	 * can apply heap_getattr().  Normally the cache control data is already
	 * valid (because the caller recently fetched the tuple via this same
	 * cache), but there are cases where we have to initialize the cache here.
	 */
	if (cacheId < 0 || cacheId >= SysCacheSize ||
		!PointerIsValid(SysCache[cacheId]))
		elog(ERROR, "invalid cache ID: %d", cacheId);
	if (!PointerIsValid(SysCache[cacheId]->cc_tupdesc))
	{
		InitCatCachePhase2(SysCache[cacheId], false);
		Assert(PointerIsValid(SysCache[cacheId]->cc_tupdesc));
	}

	return heap_getattr(tup, attributeNumber,
						SysCache[cacheId]->cc_tupdesc,
						isNull);
}

/*
 * GetSysCacheHashValue