Function get_attribute_options

src/backend/utils/cache/attoptcache.c:102–177

* get_attribute_options * Fetch attribute options for a specified table OID. */

 *		Fetch attribute options for a specified table OID.
 */
AttributeOpts *
get_attribute_options(Oid attrelid, int attnum)
{
	AttoptCacheKey key;
	AttoptCacheEntry *attopt;
	AttributeOpts *result;
	HeapTuple	tp;

	/* Find existing cache entry, if any. */
	if (!AttoptCacheHash)
		InitializeAttoptCache();
	memset(&key, 0, sizeof(key));	/* make sure any padding bits are unset */
	key.attrelid = attrelid;
	key.attnum = attnum;
	attopt =
		(AttoptCacheEntry *) hash_search(AttoptCacheHash,
										 (void *) &key,
										 HASH_FIND,
										 NULL);

	/* Not found in Attopt cache.  Construct new cache entry. */
	if (!attopt)
	{
		AttributeOpts *opts;

		tp = SearchSysCache2(ATTNUM,
							 ObjectIdGetDatum(attrelid),
							 Int16GetDatum(attnum));

		/*
		 * If we don't find a valid HeapTuple, it must mean someone has
		 * managed to request attribute details for a non-existent attribute.
		 * We treat that case as if no options were specified.
		 */
		if (!HeapTupleIsValid(tp))
			opts = NULL;
		else
		{
			Datum		datum;
			bool		isNull;

			datum = SysCacheGetAttr(ATTNUM,
									tp,
									Anum_pg_attribute_attoptions,
									&isNull);
			if (isNull)
				opts = NULL;
			else
			{
				bytea	   *bytea_opts = attribute_reloptions(datum, false);

				opts = MemoryContextAlloc(CacheMemoryContext,
										  VARSIZE(bytea_opts));
				memcpy(opts, bytea_opts, VARSIZE(bytea_opts));
			}
			ReleaseSysCache(tp);
		}