Re: Optimizing query

On Wed, Sep 9, 2026 at 6:33 PM Igor Korot <ikorot01(at)gmail(dot)com> wrote:

> Hi,
>
> On Wed, Sep 9, 2026 at 5:23 PM Ron Johnson <ronljohnsonjr(at)gmail(dot)com>
> wrote:
> >
> > On Wed, Sep 9, 2026 at 1:43 PM Igor Korot <ikorot01(at)gmail(dot)com> wrote:
> >>
> >> Hi, ALL,
> >> Can below 4 queries
> >>
> >> std::wstring query1 = L"SELECT rolname FROM pg_roles";
> >> std::wstring query2 = L"SELECT datname FROM pg_database WHERE
> >> datistemplate = true;";
> >> std::wstring query3 = L"SELECT pg_encoding_to_char( conforencoding
> >> ) AS name FROM pg_conversion";
> >> std::wstring query4 = L"SELECT collname, collencoding,
> >> collprovider collctype FROM pg_collation";
> >> std::wstring query5 = L"SELECT spcname FROM pg_tablespace";
> >>
> >> be made as one big query to use a 1 DB hit?
> >
> >
> > Bereft of the C++ cruft, here are the queries:
> > SELECT rolname
> > FROM pg_roles;
> >
> > SELECT datname
> > FROM pg_database
> > WHERE datistemplate = true;
> >
> > SELECT pg_encoding_to_char(conforencoding) AS name
> > FROM pg_conversion;
> >
> > SELECT collname, collencoding, collprovider, collctype
> > FROM pg_collation;
> >
> > SELECT spcname
> > FROM pg_tablespace;
> >
> > The sticky wicket is the four columns in the pg_collation query.
> >
> > You could probably write a stored function or procedure which returns a
> complex json object with the results of the four queries (or four jsonb
> objects, one for each query). You'd of course have to decode the json in
> your C++ program.
> >
> > Without the pg_collation query, you could make a UNION ALL like:
> > SELECT 'pg_roles' AS table_name, rolname::text AS row_value
> > FROM pg_roles
> > UNION ALL
> > SELECT 'pg_database' AS table_name, datname ::text AS row_value
> > FROM pg_database
> > WHERE datistemplate = true
> > UNION ALL
> > SELECT ' pg_conversion' AS table_name
> > , pg_encoding_to_char(conforencoding)::text AS row_value
> > FROM pg_conversion
> > UNION ALL
> > SELECT pg_tablespace AS table_name, spcname ::text AS row_value
> > FROM pg_tablespace;
> >
> > It's a lot of complication, though, for something which shouldn't be
> called very often.
>
> I was hoping to have some kind of join query. ;-)
> But you are right - the UNION ALL doesn't make sense for a
> "1-in-a-lifetime" query.
>

I could probably FULL OUTER JOIN them on the oid column. I don't know how
many rows are pg_collation, but if there are a lot, you're going to be
returning a whole lot of rows with null values, and application logic must
do something like "if *this* column is not null, then it's a role; if
*that* column
is not null, then it's a database; etc etc".

KISS and make four queries...

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