On 2016/06/21 20:42, Ashutosh Bapat wrote:
On Tue, Jun 21, 2016 at 4:36 PM, Amit Langote <langote_amit...@lab.ntt.co.jp <mailto:langote_amit...@lab.ntt.co.jp>> wrote:
On 2016/06/21 16:27, Rushabh Lathia wrote: > Now I was under impression the IS NOT NULL should be always in inverse of > IS NULL, but clearly here its not the case with wholerow. But further > looking at > the document its saying different thing for wholerow: > > https://www.postgresql.org/docs/9.5/static/functions-comparison.html > > Note: If the expression is row-valued, then IS NULL is true when the row > expression > itself is null or when all the row's fields are null, while IS NOT NULL is > true > when the row expression itself is non-null and all the row's fields are > non-null. > Because of this behavior, IS NULL and IS NOT NULL do not always return > inverse > results for row-valued expressions, i.e., a row-valued expression that > contains > both NULL and non-null values will return false for both tests. This > definition > conforms to the SQL standard, and is a change from the inconsistent behavior > exhibited by PostgreSQL versions prior to 8.2.
> And as above documentation clearly says that IS NULL and IS NOT NULL do not > always return inverse results for row-valued expressions. So need to change > the > deparse logic into postgres_fdw - how ? May be to use IS NULL rather then IS > NOT NULL? > > Input/thought?
Perhaps - NOT expr IS NULL? Like in the attached.
As the documentation describes row is NULL is going to return true when all the columns in a row are NULL, even though row itself is not null. So, with your patch a row (null, null, null) is going to be output as a NULL row. Is that right?
Yeah, I think so.
In an outer join we have to differentiate between a row being null (because there was no joining row on nullable side) and a non-null row with all column values being null. If we cast the whole-row expression to a text e.g. r.*::text and test the resultant value for nullness, it gives us what we want. A null row casted to text is null and a row with all null values casted to text is not null. postgres=# select (null, null, null)::text is not null; ?column? ---------- t (1 row) postgres=# select null::record::text is not null; ?column? ---------- f (1 row) In find_coercion_pathway(), we resort to IO coercion for record::text explicit coercion. This should always work the way we want as record_out is a strict function and for non-null values it outputs at least the parenthesis which will be treated as non-null text. 2253 /* 2254 * If we still haven't found a possibility, consider automatic casting 2255 * using I/O functions. We allow assignment casts to string types and 2256 * explicit casts from string types to be handled this way. (The 2257 * CoerceViaIO mechanism is a lot more general than that, but this is 2258 * all we want to allow in the absence of a pg_cast entry.) It would 2259 * probably be better to insist on explicit casts in both directions, 2260 * but this is a compromise to preserve something of the pre-8.3 2261 * behavior that many types had implicit (yipes!) casts to text. 2262 */
PFA the patch with the cast to text. This is probably uglier than expected, but I don't know any better test to find nullness of a record, the way we want here. The patch also includes the expected output changes in the EXPLAIN output.
How about using a system column eg, ctid, for the CASE WHEN conversion; in Rushabh's example the reference to "r1" would be converted with "CASE WHEN r1.ctid IS NOT NULL THEN ROW(r1.empno, r1.ename, r1.job, r1.mgr, r1.hiredate, r1.sal, r1.comm, r1.deptno) END". IMO I think that that would be much simpler than Ashutosh's approach.
Best regards, Etsuro Fujita -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers