PostgreSQL how to return data from anonymous code block?

I have a table mytable (id, "someLongURI", status, "userId") and a working query:

UPDATE          mytable
SET             status = 'IN_WORK'
WHERE           "someLongURI" IN (
    SELECT      "someLongURI"
    FROM        mytable
    WHERE       status = 'UNUSED'
    AND         pg_try_advisory_xact_lock(id)
    ORDER BY    id ASC 
    LIMIT       1
    FOR UPDATE 
)
RETURNING       id, "someLongURI";

But now I need to check the "userId" parameter and based on this either select existing row or update (and receive updated row). Something like that (in MySQL this would work):

IF NOT EXISTS (
    SELECT          1
    FROM            mytable tbl
    WHERE           tbl."userId" = 123
)
THEN
    UPDATE          mytable tbl
    SET             tbl.status = 'IN_WORK',
                    tbl."userId" = 123
    WHERE           tbl."someLongURI" IN (
        SELECT      "someLongURI"
        FROM        tbl.mytable
        WHERE       tbl.status = 'UNUSED'
            AND     pg_try_advisory_xact_lock(id)
        ORDER BY    id ASC 
        LIMIT       1
        FOR UPDATE 
    )
    RETURNING   tbl.id, tbl."someLongURI";
ELSE
    SELECT          tbl.id, tbl."someLongURI"
    FROM            mytable tbl
    WHERE           tbl."userId" = 123;
END IF;

I've wrapped it with DO $$ BEGIN {...} END $$; but it didn't work for me. Last thing I came to:

```
DO
RETURNS TABLE (id INT4, "someLongURI" TEXT) AS
$$
DECLARE
"_userId" INT4;
BEGIN
"_userId" = 123;

IF NOT EXISTS (
SELECT 1
FROM mytable tbl
WHERE tbl."userId" = "_userId"
)
THEN
UPDATE mytable tbl
SET tbl.status = 'IN_WORK'
tbl."userId" = "_userId"
WHERE tbl."someLongURI" IN (
SELECT "someLongURI"
FROM tbl.mytable
WHERE tbl.status = 'UNUSED'
AND pg_try_advisory_xact_lock(i

With Postgres you could do that in a single data modifying CTE without the need for an IF statement:

WITH data AS (
    SELECT          tbl.id, tbl."someLongURI"
    FROM            mytable tbl
    WHERE           tbl."userId" = 123;
), changed AS (
    UPDATE          mytable tbl
    SET             tbl.status = 'IN_WORK',
                    tbl."userId" = 123
    WHERE           tbl."someLongURI" IN (
        SELECT      "someLongURI"
        FROM        tbl.mytable
        WHERE       tbl.status = 'UNUSED'
            AND     pg_try_advisory_xact_lock(id)
        ORDER BY    id ASC 
        LIMIT       1
        FOR UPDATE 
    )
    AND NOT EXISTS (SELECT * FROM data)
    RETURNING   tbl.id, tbl."someLongURI"
)
SELECT *
FROM changed
UNION ALL
SELECT *
FROM data
WHERE NOT EXISTS (SELECT * FROM changed);

StackExchange Database Administrators Q#227844, answer score: 5