46

尝试在带有导轨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，转换为 Hstore 对我来说不是一种选择。有什么解决办法吗？

4

4 回答 4

67

这背后的原因是，在 PostgreSQL（直到 9.3）中，没有定义相等运算符json（即val1::json = val2::json总是抛出这个异常）——在 9.4 中将有一个用于该jsonb类型的运算符。

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 则不等于text。

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

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

注意：一个已知的警告DISTINCT ON：

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，这些表达式确定每个 DISTINCT ON 组中行的所需优先级。

于 2014-05-07T07:57:20.397 回答
10

对不起，我迟到了这个答案，但它可能对其他人有所帮助。

据我了解您的查询，profiles由于多对多连接integrations（您使用它来确定profiles要访问的内容），您只会获得可能的重复项。

因此，您可以使用9.1GROUP BY的新功能：

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未分组列在功能上依赖于分组列，否则将返回多个可能的值未分组的列。如果分组列（或其子集）是包含未分组列的表的主键，则存在功能依赖性。

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DISTINCT从您的SELECT条款中删除了 并添加了GROUP BY.

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

不知何故，奇妙地避免了 Postgres 对依赖列（即json在这种情况下是您的列）进行相等检查的需要。

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

于 2015-08-07T11:48:42.683 回答
6

如果您使用 PG 9.4，使用 JSONB 而不是 JSON 可以解决此问题示例：

-- 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，而在 JSON 列上失败！

还可以尝试以下操作以查看 JSONB 中的实际键是否已排序：

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 将其标识为相同记录 ：

select distinct * from t2;
3 | {"a": "1", "b": "2"}
2 | {"key": "value"}
1 | {"name": "value"}
于 2015-12-03T11:05:14.897 回答
2

是的，不幸的是，postgresjson没有实现平等，但jsonb确实如此。所以json将列迁移到jsonb它应该可以正常工作。

于 2019-07-03T09:50:01.600 回答