[MDEV-39972] Incorrect result of $[last] of JSON - Jira

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Hi,

I found that $[last] might generate an incorrect index for JSON data. This is an example:

CREATE TABLE t(

  id INT PRIMARY KEY,

  j JSON

);

INSERT INTO t VALUES

  (1, '["x","target"]'),

  (2, '["x","y","target"]');

SELECT id,

       JSON_EXTRACT(j, '$[last]') AS extracted_last,

       JSON_SEARCH(j, 'one', 'target', NULL, '$[last]') AS searched_last

FROM t

ORDER BY id;

-- 1	"target"	"$[1]"

-- 2	"y"	NULL

For the first row, the $[last] index is correct, but for the second row, the $[last] index is 1, which is incorrect.

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