JSON上的PostgreSQL索引 [英] PostgreSQL Index on JSON
问题描述
使用Postgres 9.4 ，我想在json列上创建索引，该索引将用于搜索列中的特定键.
Using Postgres 9.4, I want to create an index on a json column that will be used when searching on specific keys within the column.
例如，我有一个带有json列"animals"的农场"表.
For example I have an 'farm' table with a json column 'animals'.
动物"列具有通用格式的json对象:
The animals column has json objects of the general format:
'{"cow": 2, "chicken": 11, "horse": 3}'
我已经尝试了多个索引(分别):
I have tried a number of indexes (separately):
(1) create INDEX animal_index ON farm ((animal ->> 'cow'));
(2) create INDEX animal_index ON farm using gin ((animal ->> 'cow'));
(3) create INDEX animal_index ON farm using gist ((animal ->> 'cow'));
我想运行以下查询:
SELECT * FROM farm WHERE (animal ->> 'cow') > 3;
并使该查询使用索引.
当我运行此查询时:
SELECT * FROM farm WHERE (animal ->> 'cow') is null;
然后(1)索引有效，但我无法获得任何索引来解决不等式.
then the (1) index works, but I can't get any of the indexes to work for the inequality.
这样的索引可能吗?
服务器场表仅包含约5000个服务器场，但其中一些服务器包含100多个动物，而对于我的用例而言，查询仅花费太长时间.这样的索引是我想加快查询速度的唯一方法，但也许还有另一种选择.
The farm table contains only ~5000 farms, but some of them contain 100s of animals and the queries simply take too long for my use case. An index like this is the only method I can think of for speeding this query up, but perhaps there is another option.
推荐答案
您的其他两个索引仅因为 jsonb 获取高级索引功能.
Your other two indexes won't work simply because the ->> operator returns text, while you obviously have the jsonb gin operator classes in mind. Note that you only mention json, but you actually need jsonb for advanced indexing capabilities.
要制定最佳的索引策略，您必须更仔细地定义要覆盖的查询.您只对奶牛感兴趣吗?还是所有动物/所有标签?哪些运营商是可能的?您的JSON文档是否还包含非动物密钥?这些怎么办?您是否要在索引中包括母牛(或其他人)根本不在JSON文档中显示的行?
To work out the best indexing strategy, you'd have to define more closely which queries to cover. Are you only interested in cows? Or all animals / all tags? Which operators are possible? Does your JSON document also include non-animal keys? What to do with those? Do you want to include rows in the index where cows (or whatever) don't show up in the JSON document at all?
假设:
Assuming:
- 我们只对第一层筑巢的母牛感兴趣.
- 该值始终是有效的
integer. - 我们对没有母牛的行不感兴趣.
我建议使用功能性btree索引，就像您已经拥有的一样，但是将值强制转换为整数.我不认为您希望比较的评估结果为text(其中"2"大于"1111").
I suggest a functional btree index, much like you already have, but cast the value to integer. I don't suppose you'd want the comparison evaluated as text (where '2' is greater than '1111').
CREATE INDEX animal_index ON farm (((animal ->> 'cow')::int)); -- !
强制转换的缩写形式需要额外的括号集，以使索引表达式的语法明确.
The extra set of parentheses is required for the cast shorthand to make the syntax for the index expression unambiguous.
在查询中使用相同的表达式，以使Postgres意识到索引适用:
Use the same expression in your queries to make Postgres realize the index is applicable:
SELECT * FROM farm WHERE (animal ->> 'cow')::int > 3;
如果您需要更通用的jsonb索引，请考虑:
If you need a more generic jsonb index, consider:
对于数量众多的，已知的，静态的，琐碎的动物(如您所评论的)，我建议使用部分索引，例如:
For a known, static, trivial number of animals (like you commented), I suggest partial indexes like:
CREATE INDEX animal_index ON farm (((animal ->> 'cow')::int))
WHERE (animal ->> 'cow') IS NOT NULL;
CREATE INDEX animal_index ON farm (((animal ->> 'chicken')::int))
WHERE (animal ->> 'chicken') IS NOT NULL;
等等.
您可能必须将索引条件添加到查询中:
You may have to add the index condition to the query:
SELECT * FROM farm
WHERE (animal ->> 'cow')::int > 3
AND (animal ->> 'cow') IS NOT NULL;
看似多余，但可能是必要的.用ANALYZE进行测试！
May seem redundant, but may be necessary. Test with ANALYZE!
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