Function appendonly_fetch_init

src/backend/access/appendonly/appendonlyam.c:2365–2514

}

AppendOnlyFetchDesc
appendonly_fetch_init(Relation relation,
					  Snapshot snapshot,
					  Snapshot appendOnlyMetaDataSnapshot)
{
	AppendOnlyFetchDesc				aoFetchDesc;
	AppendOnlyStorageAttributes	   *attr;
	PGFunction					   *fns;
	StringInfoData					titleBuf;
	FormData_pg_appendonly			aoFormData;
	int								segno;

	GetAppendOnlyEntry(relation, &aoFormData);

	/*
	 * increment relation ref count while scanning relation
	 *
	 * This is just to make really sure the relcache entry won't go away while
	 * the scan has a pointer to it.  Caller should be holding the rel open
	 * anyway, so this is redundant in all normal scenarios...
	 */
	RelationIncrementReferenceCount(relation);

	/*
	 * allocate scan descriptor
	 */
	aoFetchDesc = (AppendOnlyFetchDesc) palloc0(sizeof(AppendOnlyFetchDescData));

	aoFetchDesc->relation = relation;
	aoFetchDesc->appendOnlyMetaDataSnapshot = appendOnlyMetaDataSnapshot;
	aoFetchDesc->snapshot = snapshot;

	aoFetchDesc->initContext = CurrentMemoryContext;

	aoFetchDesc->segmentFileNameMaxLen = AOSegmentFilePathNameLen(relation) + 1;
	aoFetchDesc->segmentFileName =
		(char *) palloc(aoFetchDesc->segmentFileNameMaxLen);
	aoFetchDesc->segmentFileName[0] = '\0';

	initStringInfo(&titleBuf);
	appendStringInfo(&titleBuf, "Fetch of Append-Only Row-Oriented relation '%s'",
					 RelationGetRelationName(relation));
	aoFetchDesc->title = titleBuf.data;

	/*
	 * Fill in Append-Only Storage layer attributes.
	 */
	attr = &aoFetchDesc->storageAttributes;

	/*
	 * These attributes describe the AppendOnly format to be scanned.
	 */
	if (strcmp(NameStr(aoFormData.compresstype), "") == 0 ||
		pg_strcasecmp(NameStr(aoFormData.compresstype), "none") == 0)
	{
		attr->compress = false;
		attr->compressType = "none";
	}