UPDATE in a specific order - postgresql

Question

So let's say I have a table:
SELECT * from test_table order by name;
----|----
name|ord
----|----
a |4
a |5
b |2
c |3
d |1
And I want to change the ord such that it matches the alphabetized result of the "order by name" clause. My goal, therefore, is:
SELECT * from test_table order by name;
----|----
name|ord
----|----
a |1
a |2
b |3
c |4
d |5
Is there a good way in Postgres to do this? I have a new sequence I can pull from, I'm just not sure how to do this cleanly in-place, or if that's even possible. Or should I just store the results of the selection, then iterate over and select each name, assigning a new ord value to them? (They all have unique IDs, so the repeat shouldn't matter)

Answer 1

You don't need any sequence for this.
The first step is determinate the new data:
SELECT
*
FROM test_table AS test_table_old
LEFT JOIN (
SELECT
*, row_number() OVER () AS ord_new
FROM test_table
ORDER BY name, ord
) AS test_table_new USING (name, ord)
;
Then convert this to an update:
UPDATE test_table SET
ord = test_table_new.ord_new
FROM test_table AS test_table_old
LEFT JOIN (
SELECT
*, row_number() OVER () AS ord_new
FROM test_table
ORDER BY name, ord
) AS test_table_new USING (name, ord)
WHERE (test_table.name, test_table.ord) = (test_table_old.name, test_table_old.ord)
;
If you need a new sequence, then replace "row_numer() OVER ()" to "nextval('the_new_sequence_name')".