From b41645460af563cfd4e4f57f354058cf69ef3b14 Mon Sep 17 00:00:00 2001 From: Alexander Korotkov Date: Fri, 29 Jan 2021 15:27:55 +0300 Subject: [PATCH] Document behavior of the .** jsonpath accessor in the lax mode When the .** jsonpath accessor handles the array, it selects both array and each of its elements. When using lax mode, subsequent accessors automatically unwrap arrays. So, the content of each array element may be selected twice. Even though this behavior is counterintuitive, it's correct because everything works as designed. This commit documents it. Backpatch to 12 where the jsonpath language was introduced. Reported-by: Thomas Kellerer Bug: #16828 Discussion: https://postgr.es/m/16828-2b0229babfad2d8c%40postgresql.org Discussion: https://postgr.es/m/CAPpHfdtS-nNidT%3DEqZbAYOPcnNOWh_sd6skVdu2CAQUGdvpT8Q%40mail.gmail.com Author: Alexandex Korotkov, revised by Tom Lane Reviewed-by: Alvaro Herrera, Thomas Kellerer, Tom Lane Backpatch-through: 12 --- doc/src/sgml/func.sgml | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 4342c8e74f..de1b1b6deb 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -16277,6 +16277,24 @@ strict $.track.segments[*].location + + The .** accessor can lead to surprising results + when using the lax mode. For instance, the following query selects every + HR value twice: + +lax $.**.HR + + This happens because the .** accessor selects both + the segments array and each of its elements, while + the .HR accessor automatically unwraps arrays when + using the lax mode. To avoid surprising results, we recommend using + the .** accessor only in the strict mode. The + following query selects each HR value just once: + +strict $.**.HR + + +