Function SetNextFileSegForRead

src/backend/access/appendonly/appendonlyam.c:141–326

* Open the next file segment to scan and allocate all resources needed for it. */

 * Open the next file segment to scan and allocate all resources needed for it.
 */
bool
SetNextFileSegForRead(AppendOnlyScanDesc scan)
{
	Relation	reln = scan->aos_rd;
	int			segno = -1;
	int64		eof = 0;
	int			formatversion = -2; /* some invalid value */
	bool		finished_all_files = true;	/* assume */
	int32		fileSegNo;
	bool 		isParallel = false;
	ParallelBlockTableScanDesc pbscan = NULL;

	if (scan->rs_base.rs_parallel != NULL)
	{
		isParallel = true;
		pbscan = (ParallelBlockTableScanDesc) scan->rs_base.rs_parallel;
	}


	Assert(scan->aos_need_new_segfile); /* only call me when last segfile
										 * completed */
	Assert(!scan->aos_done_all_segfiles);	/* don't call me if I told you to
											 * stop */


	if (!scan->initedStorageRoutines)
	{
		PGFunction *fns = NULL;
		RelationOpenSmgr(reln);

		AppendOnlyStorageRead_Init(
								   &scan->storageRead,
								   scan->aoScanInitContext,
								   scan->usableBlockSize,
								   NameStr(scan->aos_rd->rd_rel->relname),
								   RelationGetRelid(scan->aos_rd),
								   scan->title,
								   &scan->storageAttributes,
								   &scan->aos_rd->rd_node, reln->rd_smgr->smgr_ao);

		/*
		 * There is no guarantee that the current memory context will be
		 * preserved between calls, so switch to a safe memory context for
		 * retrieving compression information.
		 */
		MemoryContext oldMemoryContext = MemoryContextSwitchTo(scan->aoScanInitContext);

		/* Get the relation specific compression functions */

		fns = get_funcs_for_compression(scan->storageAttributes.compressType);
		scan->storageRead.compression_functions = fns;

		if (scan->storageRead.compression_functions != NULL)
		{
			PGFunction	cons = fns[COMPRESSION_CONSTRUCTOR];
			CompressionState *cs;
			StorageAttributes sa;