PostgreSQL嵌套JSON查询 [英] PostgreSQL Nested JSON Querying

问题描述

在PostgreSQL 9.3.4上，我有一个名为"person"的JSON类型列，并且其中存储的数据的格式为{dogs: [{breed: <>, name: <>}, {breed: <>, name: <>}]}.我想检索索引为0的那只狗.这是我运行的两个查询:

On PostgreSQL 9.3.4, I have a JSON type column called "person" and the data stored in it is in the format {dogs: [{breed: <>, name: <>}, {breed: <>, name: <>}]}. I want to retrieve the breed of dog at index 0. Here are the two queries I ran:

不起作用

db=> select person->'dogs'->>0->'breed' from people where id = 77;
ERROR:  operator does not exist: text -> unknown
LINE 1: select person->'dogs'->>0->'bree...
                                 ^
HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.

工程

select (person->'dogs'->>0)::json->'breed' from es_config_app_solutiondraft where id = 77;
 ?column?
-----------
 "westie"
(1 row)

为什么必须进行类型转换?效率低下吗?我是在做错什么，还是对Postgres JSON支持有必要?

Why is the type casting necessary? Isn't it inefficient? Am I doing something wrong or is this necessary for postgres JSON support?

推荐答案

这是因为运算符->>获取JSON数组元素作为文本.您需要强制转换以将其结果转换回JSON.

This is because operator ->> gets JSON array element as text. You need a cast to convert its result back to JSON.

您可以使用运算符->消除这种多余的转换:

You can eliminate this redundant cast by using operator ->:

select person->'dogs'->0->'breed' from people where id = 77;

这篇关于PostgreSQL嵌套JSON查询的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！