Postgresql JSON 数据列不同

Question

尝试在带有 rails 的模式上做不同的事情。

2.1.1 :450 > u.profiles.select("profiles.*").distinct


Profile Load (0.9ms)  SELECT DISTINCT profiles.* FROM "profiles" INNER JOIN "integration_profiles" ON "profiles"."id" = "integration_profiles"."profile_id" INNER JOIN "integrations" ON "integration_profiles"."integration_id" = "integrations"."id" WHERE "integrations"."user_id" = $1  [["user_id", 2]]
PG::UndefinedFunction: ERROR:  could not identify an equality operator for type json
LINE 1: SELECT DISTINCT profiles.* FROM "profiles" INNER JOIN "integ...
                        ^
: SELECT DISTINCT profiles.* FROM "profiles" INNER JOIN "integration_profiles" ON "profiles"."id" = "integration_profiles"."profile_id" INNER JOIN "integrations" ON "integration_profiles"."integration_id" = "integrations"."id" WHERE "integrations"."user_id" = $1
ActiveRecord::StatementInvalid: PG::UndefinedFunction: ERROR:  could not identify an equality operator for type json
LINE 1: SELECT DISTINCT profiles.* FROM "profiles" INNER JOIN "integ...
                        ^
: SELECT DISTINCT profiles.* FROM "profiles" INNER JOIN "integration_profiles" ON "profiles"."id" = "integration_profiles"."profile_id" INNER JOIN "integrations" ON "integration_profiles"."integration_id" = "integrations"."id" WHERE "integrations"."user_id" = $1
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/rack-mini-profiler-0.9.1/lib/patches/sql_patches.rb:109:in `prepare'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/rack-mini-profiler-0.9.1/lib/patches/sql_patches.rb:109:in `prepare'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/postgresql_adapter.rb:834:in `prepare_statement'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/postgresql_adapter.rb:795:in `exec_cache'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:139:in `block in exec_query'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/abstract_adapter.rb:442:in `block in log'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activesupport-4.0.4/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/abstract_adapter.rb:437:in `log'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/postgresql/database_statements.rb:137:in `exec_query'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/postgresql_adapter.rb:908:in `select'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/abstract/database_statements.rb:32:in `select_all'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/connection_adapters/abstract/query_cache.rb:63:in `select_all'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/querying.rb:36:in `find_by_sql'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/relation.rb:585:in `exec_queries'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/association_relation.rb:15:in `exec_queries'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/relation.rb:471:in `load'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/relation.rb:220:in `to_a'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/activerecord-4.0.4/lib/active_record/relation.rb:573:in `inspect'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/railties-4.0.4/lib/rails/commands/console.rb:90:in `start'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/railties-4.0.4/lib/rails/commands/console.rb:9:in `start'
    from /Users/mmahalwy/.rvm/gems/ruby-2.1.1/gems/railties-4.0.4/lib/rails/commands.rb:62:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'2.1.1 :451 > 

收到错误PG::UndefinedFunction: ERROR: could not identify an equality operator for type json

在这种情况下，我不能选择转换为 Hstore。有什么解决办法吗？

Answer 1

这背后的原因是，在 PostgreSQL（最高 9.3）中没有为json 定义相等运算符（即val1::json = val2::json 总是会抛出这个异常）——在 9.4 中将有一个用于@987654324 @类型。

一种解决方法是，您可以将json 字段转换为text。但这不会涵盖所有 json 等式。前任{"a":1,"b":2} 应该等于 {"b":2,"a":1}，但如果转换为 text，则不会相等。

另一种解决方法是（如果您有该表的主键 - 应该是）您可以使用 DISTINCT ON (<expressions>) form：

u.profiles.select("DISTINCT ON (profiles.id) profiles.*")

注意：DISTINCT ON 的一个已知警告：

DISTINCT ON 表达式必须匹配最左边的 ORDER BY 表达式。 ORDER BY 子句通常包含附加表达式，用于确定每个 DISTINCT ON 组中行的所需优先级。

Answer 2

对不起，我迟到了这个答案，但它可能对其他人有所帮助。

据我了解，您只能在 profiles 上获得可能的重复项，因为与 integrations 的多对多连接（您用于确定要访问哪个 profiles）。

因此，您可以使用新的GROUP BY 功能as of 9.1：

当 GROUP BY 存在时，SELECT 列表表达式引用未分组列是无效的，除非在聚合函数中 或者如果未分组列在功能上依赖于分组列，否则会有超过为未分组的列返回的一个可能值。如果分组列（或其子集）是包含未分组列的表的主键，则存在函数依赖关系。

所以在你的情况下，你可以让 Ruby 来创建查询（对不起，我不知道你正在使用的 Ruby 语法）......

SELECT profiles.* 
FROM "profiles" 
  INNER JOIN "integration_profiles" ON "profiles"."id" = "integration_profiles"."profile_id" 
  INNER JOIN "integrations" ON "integration_profiles"."integration_id" = "integrations"."id" 
WHERE "integrations"."user_id" = $1
GROUP BY "profiles"."id"

我只从您的SELECT 子句中删除了DISTINCT 并添加了GROUP BY。

通过仅引用GROUP BY 中的id，您可以利用该新功能，因为所有剩余的profiles 列在“功能上依赖于”该id 主键。

不知何故，这避免了 Postgres 需要对依赖列进行相等性检查（即在这种情况下是您的 json 列）。

DISTINCT ON 解决方案也很棒，在您的情况下显然足够了，但是您不能使用像 array_agg 这样的聚合函数。您可以使用这种GROUP BY 方法。快乐的时光！ :)

Answer 3

如果你使用 PG 9.4 ，使用 JSONB 而不是 JSON 可以解决这个问题 示例：

-- JSON datatype test 

create table t1 (id int, val json);
insert into t1 (id,val) values (1,'{"name":"value"}');
insert into t1 (id,val) values (1,'{"name":"value"}');
insert into t1 (id,val) values (2,'{"key":"value"}');
select * from t1 order by id;
select distinct * from t1 order by id;

-- JSONB datatype test 

create table t2 (id int, val jsonb);
insert into t2 (id,val) values (1,'{"name":"value"}');
insert into t2 (id,val) values (1,'{"name":"value"}');
insert into t2 (id,val) values (2,'{"key":"value"}');

select * from t2 order by id;

select distinct * from t2 order by id;

Result of running the above script :

CREATE TABLE
INSERT 0 1
INSERT 0 1
INSERT 0 1
1 | {"name":"value"}
1 | {"name":"value"}
2 | {"key":"value"}

ERROR:  could not identify an equality operator for type json
LINE 1: select distinct * from t1 order by id;
                    ^
CREATE TABLE
INSERT 0 1
INSERT 0 1
INSERT 0 1
1 | {"name": "value"}
1 | {"name": "value"}
2 | {"key": "value"}

1 | {"name": "value"}
2 | {"key": "value"}

如您所见，PG 成功地在 JSONB 列上暗示 DISTINCT 虽然它在 JSON 列上失败！

还可以尝试以下操作以查看 JSONB 中的键是否已排序：

insert into t2 values (3, '{"a":"1", "b":"2"}');
insert into t2 values (3, '{"b":"2", "a":"1"}');
select * from t2;

1 | {"name": "value"}
1 | {"name": "value"}
2 | {"key": "value"}
3 | {"a": "1", "b": "2"}
3 | {"a": "1", "b": "2"}

注意 '{"b":"2", "a":"1"}' 被插入为 '{"a":"1", "b":"2"}' 因此 PG 将其识别为同一记录：

select distinct * from t2;
3 | {"a": "1", "b": "2"}
2 | {"key": "value"}
1 | {"name": "value"}

Answer 4

是的，不幸的是 postgres json 没有实现平等，但 jsonb 做到了。所以将json 列迁移到jsonb 应该可以正常工作。