2021年4月8日(木) 15:04 Fujii Masao <masao.fu...@oss.nttdata.com>: > > On 2021/04/08 13:43, Kohei KaiGai wrote: > > In case when a local table (with no children) has same contents, > > TRUNCATE command > > witll remove the entire table contents. > > But if there are local child tables that inherit the local parent table, and > TRUNCATE ONLY <parent table> is executed, only the contents in the parent > will be truncated. I was thinking that this behavior should be applied to the > foreign table whose remote (parent) table have remote child tables. > > So what we need to reach the consensus is; how far ONLY option affects. > Please imagine the case where we have > > (1) local parent table, also foreign table of remote parent table > (2) local child table, inherits local parent table > (3) remote parent table > (4) remote child table, inherits remote parent table > > I think that we agree all (1), (2), (3) and (4) should be truncated if local > parent table (1) is specified without ONLY in TRUNCATE command. OTOH, if ONLY > is specified, we agree that at least local child table (2) should NOT be > truncated. > My understanding of a foreign table is a representation of external data, including remote RDBMS but not only RDBMS, regardless of the parent-child relationship at the local side. So, once a local foreign table wraps entire tables tree (a parent and relevant children) at the remote side, at least, it shall be considered as a unified data chunk from the standpoint of the local side.
Please assume if file_fdw could map 3 different CSV files, then truncate on the foreign table may eliminate just 1 of 3 files. Is it an expected / preferable behavior? Basically, we don't assume any charasteristics of the data on behalf of the FDW driver, even if it is PostgreSQL server. Thus, I think the new API will expect to eliminate the entire rows on behalf of the foreign table, regardless of the ONLY-clause, because it already controls which foreign-tables shall be picked up, but does not control which part of the foreign table shall be eliminated. > So the remaining point is; remote tables (3) and (4) should be truncated or > not when ONLY is specified? You seem to argue that both should be truncated > by removing extra list. I was thinking that only remote parent table (3) > should be truncated. That is, IMO we should treat the truncation on foreign > table as the same as that on its forein data source. > > Other people might think neither (3) nor (4) should be truncated in that case > because ONLY should affect only the table directly specified in TRUNCATE > command, i.e., local parent table (1). For now this also looks good to me. > In case when the local foreign table is a parent, the entire remote table shall be truncated, if ONLY is given. In case when the local foreign table is a child, nothing shall be happen (API is not called), if ONLY is given. IMO, it is stable and simple definition, even if FDW driver wraps non-RDBMS data source that has no idea of table inheritance. Best regards, -- HeteroDB, Inc / The PG-Strom Project KaiGai Kohei <kai...@heterodb.com>
