Thread: Convert pg_constraint.conkey array to same-order array of column names

On 3/23/23 04:12, Dominique Devienne wrote:
> We have a query returning 1 row per constraint column,
> which until recently we didn't realize wasn't preserving order of the 
> columns.
> 
> A colleague fixed that, with something like below:
> 
> SELECT ...
> FROM pg_catalog.pg_constraint cnstr
> ...
> CROSS JOIN LATERAL UNNEST(cnstr.conkey) WITH ORDINALITY AS cols(value, rank)
>   ORDER BY cols.rank
> 
> But I'm wondering about getting 1 row per constraint instead,
> and fetching an array of column names.
> 
> So is there a way to "convert" int2[] conkey array into a text[] of 
> those column names?

A before coffee solution:

WITH ck AS (
     SELECT
         conrelid,
         unnest(conkey) AS ky
     FROM
         pg_constraint
     WHERE
         conrelid = 'cell_per'::regclass
)
SELECT
     array_agg(attname)
FROM
     ck
JOIN
     pg_attribute
ON
     attnum = ck.ky AND ck.conrelid = pg_attribute.attrelid;

array_agg
-----------------------------
{category,line_id,category}


-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 2023-Mar-23, Dominique Devienne wrote:

> We have a query returning 1 row per constraint column,
> which until recently we didn't realize wasn't preserving order of the
> columns.
> 
> A colleague fixed that, with something like below:
> 
> SELECT ...
> FROM pg_catalog.pg_constraint cnstr
> ...
> CROSS JOIN LATERAL UNNEST(cnstr.conkey) WITH ORDINALITY AS cols(value, rank)
>  ORDER BY cols.rank
> 
> But I'm wondering about getting 1 row per constraint instead,
> and fetching an array of column names.

IIRC you can use array_agg(unnest ORDER BY ordinality), grouping by
constraint OID or name.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"La espina, desde que nace, ya pincha" (Proverbio africano)

On 3/24/23 03:28, Dominique Devienne wrote:
> On Thu, Mar 23, 2023 at 4:20 PM Adrian Klaver <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>> wrote:
> 
>     On 3/23/23 04:12, Dominique Devienne wrote:
>      > CROSS JOIN LATERAL UNNEST(cnstr.conkey) WITH ORDINALITY AS
>     cols(value, rank)
>      >   ORDER BY cols.rank
>     A before coffee solution:
> 
> 
> Thanks for answering Adrian. And sorry for the delay in responding.
> 
>     WITH ck AS (
>           SELECT
>               conrelid,
>               unnest(conkey) AS ky
>           FROM
>               pg_constraint
>           WHERE
>               conrelid = 'cell_per'::regclass
>     )
> 
> 
> This part surprised me. I didn't know a table-valued function could be 
> used like this on the select-clause.
> 
> Both queries below yield the same rows for me, in the same order:
> 
> => select conname, unnest(conkey), conrelid::regclass::text from 
> pg_constraint where conrelid::regclass::text like ... and 
> cardinality(conkey) = 8;
> => select conname, key.value, conrelid::regclass::text from 
> pg_constraint cross join lateral unnest(conkey) as key(value) where 
> conrelid::regclass::text like ... and cardinality(conkey) = 8;
> 
> So your compact form is equivalent to the second form?
> What about the order? Is it guaranteed?
> I was "raised" on the "order is unspecified w/o an order-by-clause". Why 
> would be it be different here?
> In our case, the query is more complex, with joins on pg_namespace, 
> pg_class, and pg_attribute, on
> all constraints of a schema, and the order came out wrong w/o adding 
> WITH ORDINALITY and ordering on it.

Your original question was:

"But I'm wondering about getting 1 row per constraint instead,
and fetching an array of column names.

So is there a way to "convert" int2[] conkey array into a text[] of those
column names?"

That is what I showed as a simple example.

> 
> Thus I worry the order is plan-dependent, and not guaranteed. Am I wrong 
> to worry?
> The form you provide seems no different from our old form, to my 
> non-expert eye. --DD

-- 
Adrian Klaver
adrian.klaver@aklaver.com

On 3/24/23 9:44 AM, Dominique Devienne wrote:
> On Fri, Mar 24, 2023 at 5:40 PM Adrian Klaver <adrian.klaver@aklaver.com 
> <mailto:adrian.klaver@aklaver.com>> wrote:
> 
>     On 3/24/23 03:28, Dominique Devienne wrote:
>      > On Thu, Mar 23, 2023 at 4:20 PM Adrian Klaver
>     <adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>
>      > <mailto:adrian.klaver@aklaver.com
>     <mailto:adrian.klaver@aklaver.com>>> wrote:
>     Your original question was:
> 
>     "But I'm wondering about getting 1 row per constraint instead,
>     and fetching an array of column names.
> 
>     So is there a way to "convert" int2[] conkey array into a text[] of
>     those
>     column names?"
> 
>     That is what I showed as a simple example.
> 
> 
> Sorry, didn't mean to offend you.

There was no offense taken, it was about confusion as to what was being 
asked?

> 
> My post started with "preserving order of the columns",
> but I probably didn't make it clear enough preserving the source array's 
> order was also a requirement.


-- 
Adrian Klaver
adrian.klaver@aklaver.com