How can I create a trigger to unpack json data? - sql

Question

This is the simplest reproducible example I can come up with. I would like to create a materialized view which is managed by triggers. This MV extracts some data from a json column, which could have an unbounded number of values in an array. Since we don't yet have json_table I am extracting these in a recursive CTE.
create or replace table base_data(
id int auto_increment not null primary key,
record_data longtext not null check (json_valid(record_data))
);
insert into base_data(record_data)
values
('{"x":1, "y":[2,3,4]}')
;
create or replace table mv_integers as
with recursive array_values as (
select
json_extract(b.record_data, '$.y[*]') val
, b.id
, json_length(json_extract(b.record_data, '$.y[*]')) n
from base_data b
), rec_elems as (
select av.id, 0 as i, json_extract(av.val, '$[0]') elem
from array_values av
where av.n > 0
union all
select av.id, e.i + 1, json_extract(av.val, concat('$[', e.i + 1, ']'))
from array_values av
inner join rec_elems e on av.id = e.id
where (i + 1) < av.n
)
select r.id record_id, r.elem extracted_integer, 'y' as 'comes_from'
from rec_elems r
union all
select b.id, json_value(b.record_data, '$.x'), 'x'
from base_data b
;
create or replace trigger maintain_mv_integers
after insert on base_data for each row
insert mv_integers(record_id, extracted_integer, comes_from)
with recursive array_values as (
select
json_extract(new.record_data, '$.y[*]') val
, new.id
, json_length(json_extract(new.record_data, '$.y[*]')) n
), rec_elems as (
select av.id, 0 as i, json_extract(av.val, '$[0]') elem
from array_values av
where av.n > 0
union all
select av.id, e.i + 1, json_extract(av.val, concat('$[', e.i + 1, ']'))
from array_values av
inner join rec_elems e on av.id = e.id
where (i + 1) < av.n
)
select r.id record_id, r.elem extracted_integer, 'y' as 'comes_from'
from rec_elems r
union all
select new.id, json_value(new.record_data, '$.x'), 'x'
;
insert into base_data(record_data)
values
('{"x":5, "y":[6,7,4]}')
;
What the code does:
Create a table
Populate that table with some data
Create a materialized view from that data
Create a trigger to maintain the materialized view going forward
Insert some new data to see if the trigger works
The code fails on the last step. The error is Unknown column 'record_data' in 'NEW'. If I comment out the bit before the union all and just leave the last bit which doesn't involve a CTE, then the code works. Does this mean that CTEs are not supported in triggers? I hope not and I can't find any documentation that supports this conclusion.
My version is 10.5.8-MariaDB-1:10.5.8+maria~bionic running on WSL2 Ubuntu.
So how to get this to work?

Answer 1

I have come up with a workaround. All I did was put the logic of the trigger into a stored procedure and call it from the trigger. That fact that this works and the original does not, suggests that this is either a bug or undocumented behavior.
UPDATE: this is now a confirmed bug.
create or replace table base_data(
id int auto_increment not null primary key,
record_data longtext not null check (json_valid(record_data))
);
insert into base_data(record_data)
values
('{"x":1, "y":[2,3,4]}')
;
create or replace table mv_integers as
with recursive array_values as (
select
json_extract(b.record_data, '$.y[*]') val
, b.id
, json_length(json_extract(b.record_data, '$.y[*]')) n
from base_data b
), rec_elems as (
select av.id, 0 as i, json_extract(av.val, '$[0]') elem
from array_values av
where av.n > 0
union all
select av.id, e.i + 1, json_extract(av.val, concat('$[', e.i + 1, ']'))
from array_values av
inner join rec_elems e on av.id = e.id
where (i + 1) < av.n
)
select r.id record_id, r.elem extracted_integer, 'y' as 'comes_from'
from rec_elems r
union all
select b.id, json_value(b.record_data, '$.x'), 'x'
from base_data b
;
create or replace procedure sp_maintain_mv_integers(
in record_id int unsigned,
in json_data longtext
)
insert mv_integers(record_id, extracted_integer, comes_from)
with recursive array_values as (
select
json_extract(json_data, '$.y[*]') val
, record_id id
, json_length(json_extract(json_data, '$.y[*]')) n
), rec_elems as (
select av.id, 0 as i, json_extract(av.val, '$[0]') elem
from array_values av
where av.n > 0
union all
select av.id, e.i + 1, json_extract(av.val, concat('$[', e.i + 1, ']'))
from array_values av
inner join rec_elems e on av.id = e.id
where (i + 1) < av.n
)
select r.id record_id, r.elem extracted_integer, 'y' as 'comes_from'
from rec_elems r
union all
select record_id, json_value(json_data, '$.x'), 'x';
create or replace trigger tg_maintain_mv_integers
after insert on base_data for each row
call sp_maintain_mv_integers(new.id, new.record_data);
insert into base_data(record_data)
values
('{"x":5, "y":[6,7,4]}')
;