Hi!
Let's consider some examples.
# select '[1,2]'::jsonb @> '[1,2,2]'::jsonb;
?column?
----------
f
(1 row)
One may think it's because second jsonb array contain two "2". So, contains takes care about count of equal elements.
# select '[1,1,2]'::jsonb @> '[1,2,2]'::jsonb;
?column?
----------
t
(1 row)
But, it's not. Jsonb contains takes care only about length of array.
# select '[[1,2]]'::jsonb @> '[[1,2,2]]'::jsonb;
?column?
----------
t
(1 row)
Even more weird :)
The reason why jsonb contains behaves so is check in the beginning of jsonb_contains. It makes fast check of jsonb type and elements count before calling JsonbDeepContains.
if (JB_ROOT_COUNT(val) < JB_ROOT_COUNT(tmpl) ||
JB_ROOT_IS_OBJECT(val) != JB_ROOT_IS_OBJECT(tmpl))
PG_RETURN_BOOL(false);
It's likely that "JB_ROOT_COUNT(val) < JB_ROOT_COUNT(tmpl)" should be checked only for objects, not arrays. Also, should JsonbDeepContains does same fast check when it deals with nested objects?
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With best regards,
Alexander Korotkov.