BUG #16389: OOM on CTE since version 12

The following bug has been logged on the website:

Bug reference:      16389
Logged by:          Rikard Pavelic
Email address:      rikard@ngs.hr
PostgreSQL version: 12.2
Operating system:   Centos 7, Linux 3.10
Description:

Hi, we are testing upgrade from 11 to 12 and encountered some strange
OOMs.
This one was consistently happening on several instances, while it passed
normally on many other ones:

org.postgresql.util.PSQLException:ERROR: out of memory
  Detail: Failed on request of size 52 in memory context "ExecutorState".
Where: "WITH source AS (
    SELECT 
        i,
        row_number() over () as ord,
        (t.old).amount_array AS old,
        (t.new).amount_array AS new,
        coalesce(array_upper((t.old).amount_array, 1), 0) AS old_count,
        coalesce(array_upper((t.new).amount_array, 1), 0) AS new_count,
        (SELECT array_agg(row(_r.pk1, _r.pk2)) FROM unnest((t.old).amount_array)
_r) as old_pks,
        (SELECT array_agg(row(_r.pk1, _r.pk2)) FROM unnest((t.new).amount_array)
_r) as new_pks,
        (SELECT array_agg(i) FROM generate_series(1, CASE WHEN
coalesce(array_upper((t.old).amount_array, 1), 0) >
coalesce(array_upper((t.new).amount_array, 1), 0) THEN
array_upper((t.old).amount_array, 1) ELSE array_upper((t.new).amount_array,
1) END) i) as indexes
    FROM global_temp_input_table t
    WHERE 
        NOT (t.old).amount_array IS NULL AND (t.new).amount_array IS NULL
        OR (t.old).amount_array IS NULL AND NOT (t.new).amount_array IS NULL
        OR NOT (t.old).amount_array IS NULL AND NOT (t.new).amount_array IS NULL
AND (t.old).amount_array != (t.new).amount_array
)
INSERT INTO global_temp_update
SELECT i, index, old[index] AS old, 
    CASE 
        WHEN old_pks[index] = new_pks[index] THEN new[index] 
        WHEN NOT COALESCE(old_pks[index] = ANY (new_pks), false) THEN null
        ELSE (select n from unnest(new) n where n.pk1 = old[index].pk1 AND n.pk2 =
old[index].pk2)
    END AS changed,
    new[index] AS new,
    CASE WHEN old_pks[index] = new_pks[index] THEN false ELSE NOT
new_pks[index] IS NULL AND NOT COALESCE(new_pks[index] = ANY (old_pks),
false) END AS is_new
FROM
(
    SELECT i as _i, ord as _ord, unnest((SELECT array_agg(i) FROM
generate_series(1, CASE WHEN old_count > new_count THEN old_count ELSE
new_count END) i)) as index
    FROM source s
) ix
INNER JOIN source sq ON ix._i = sq.i AND ix._ord = sq.ord"

The global input temp tables look like this

CREATE UNLOGGED TABLE global_temp_input_table
(
    i integer,
    old data_source_view,
    new data_source_view
);

where data_source_view is an view with an array of composites (amount_array)
which gets unnested and compared here.

While the query is not optimal, the OOMs stopped after we put MATERIALIZED
on top (which does not seem necessary as it's used twice).
Anyway, it's not an immediate problem for us anymore, as we've rewritten it
into a saner form,
but it hints at some underlying issue with recent CTE optimization.

Regards,
Rikard