Re: Postgres_fdw join pushdown - wrong results with whole-row reference - Mailing list pgsql-hackers
| From | Ashutosh Bapat |
|---|---|
| Subject | Re: Postgres_fdw join pushdown - wrong results with whole-row reference |
| Date | |
| Msg-id | CAFjFpRerFZ=rNF18gxUAJT--M+-yBy7Y4zUa=U=L6qDgeL9TMg@mail.gmail.com Whole thread Raw |
| In response to | Re: Postgres_fdw join pushdown - wrong results with whole-row reference (Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>) |
| Responses |
Re: Postgres_fdw join pushdown - wrong results with
whole-row reference
Re: Postgres_fdw join pushdown - wrong results with whole-row reference |
| List | pgsql-hackers |
On Tue, Jun 21, 2016 at 4:36 PM, Amit Langote <Langote_Amit_f8@lab.ntt.co.jp> wrote:
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.
Perhaps - NOT expr IS NULL? Like in the attached.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?
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?
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 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 */
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.
--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company
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