How to call a function using the results of a query ordered - sql

Question

I'm trying to call a function on each of the values that fit my query ordered by date. The reason being that the (black box) function is internally aggregating values into a string and I need the aggregated values to be in the timestamp order.
The things that I have tried are below: (function returns a boolean value and does blackbox things that I do not know and cannot modify)
-- This doesn't work
SELECT
bool_and (
function(MT.id)
)
FROM my_table MT
WHERE ...conditions...
ORDER BY MT.timestamp_value, MT.id
and got the error column "mt.timestamp_value" must appear in the GROUP BY clause or be used in an aggregate function. If I remove the ORDER BY as below, it will also work:
-- This works!
SELECT
bool_and (
function(MT.id)
)
FROM my_table MT
WHERE ...conditions...
I also tried removing the function and only selected MT.id and it worked, but with the function it doesn't. So I tried using the GROUP BY clause.
Doing that, I tried:
-- This also doesn't work
SELECT
bool_and(
function(MT.id)
)
FROM my_table MT
WHERE ...conditions...
GROUP BY MT.id, MT.timestamp_value
ORDER BY MT.timestamp_value, MT.id
but this gives the error more than one row returned by a subquery used as an expression. MT.id is the primary key btw. It also works without the function and just SELECT MT.id
Ideally, a fix to either one of the code bits above would be nice or otherwise something that fulfills the following:
-- Not real postgresql code but something I want it to do
SELECT FUNCTION(id)
FOR EACH id in (MY SELECT STATEMENT HERE ORDERED)
In response to #a_horse_with_no_name
This code falls under a section of another query that looks like the below:
SELECT Function2()
WHERE true = (
(my_snippet)
AND (...)
AND (...)
...
)

Answer 1

The error is clear. The subquery SELECT function(MT.id) is returning more than 1 row and the calling function bool_and can only operate on 1 row at a time. Adjust the subquery so that it only returns 1 record.

Answer 2

Issue resolution:
I discovered the reason that everything was failing was because of the AND in
WHERE true = (
(my_snippet)
AND (...)
AND (...)
...
)
What happened was that using GROUP BY and using ORDER BY caused the value returned by my snippet to be multiple rows of true.
Without the GROUP BY and the ORDER BY, it only returned a single row of true.
So my solution was to wrap the code into another bool_and and use that.
SELECT
bool_and(vals)
FROM(
SELECT
bool_and(
function(MT.id)
) as vals
FROM my_table MT
WHERE ...conditions...
GROUP BY MT.id, MT.timestamp_value
ORDER BY MT.timestamp_value, MT.id
) t

Answer 3

Since I have to guess the reason, and the way that you are trying to accomplish your stated goal:
"return value of the function is aggregated into a string"
And since:
You are using: bool_and and therefore the return value of the function must be boolean, and
The only aggregation I can see is the bool_and aggregation into either true or false, and
You mention that the function is a black box to you,
I would presume that instead of:
"return value of the function is aggregated into a string"
You meant to say: function is aggregating (input/transformed input) values into a string,
And you need this aggregating to be in a certain order.
Further I assume that you own the my_table and can create indexes on it.
So, if you need the function being used in the context:
bool_and ( function(MT.id) )
to process (and therefore aggregate into string) MT.id inputs (or their transformed values) in a certain order, you need to create a clustered index in that order for your my_table table.
To accomplish that in postgresql, you need to (instead of using the group by and order by):
create that_index, in the order you need for the aggregation, for your my_table table, and then
run: CLUSTER my_table USING that_index to physically bake in that order to table structure, and therefore ensure the default aggregation order to be in that order in the bool_and ( function(MT.id) ) aggregation.
(see CLUSTER for more info)