Question

尝试在使用rails的模式上做区别。

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不是我的选择。有什么工作吗？

Answer 1

这背后的原因是，在PostgreSQL（最多9.3）中没有为json定义的等式运算符（即val1::json = val2::json将始终抛出此异常） - 在9.4中将有一个对于jsonb类型。

一种解决方法是，您可以将json字段转换为text。但这并不能涵盖所有json的平等。 f.ex. {"a":1,"b":2}应该等于{"b":2,"a":1}，但如果投放到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表达式必须与最左边的ORDER BY表达式匹配。 ORDER BY子句通常包含其他表达式，用于确定每个DISTINCT ON组中行的所需优先级。

Answer 2

对不起，我对这个答案迟到了，但它可能对其他人有所帮助。

据我了解您的查询，您只能在profiles上获得重复项，因为与integrations的多对多联接（您用它来确定哪个profiles访问）。

因此，您可以使用新的GROUP BY功能as of 9.1：

当存在GROUP BY时，SELECT列表表达式无法引用未聚合的列，但聚合函数中或未分组的列在功能上依赖于分组列，因为否则会超过为未分组的列返回一个可能的值。如果分组列（或其子集）是包含未分组列的表的主键，则存在功能依赖性。

所以在你的情况下，你可以让Ruby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DISTINCT子句中删除了SELECT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列在该ID主键上都是“功能相关的”。

不知何故，奇妙地避免了Postgres对依赖列（即本例中的json列）进行等式检查的需要。

DISTINCT ON解决方案也很棒，在您的情况下显然足够了，但您不能使用array_agg之类的聚合函数。您可以使用此GROUP BY方法。快乐的时光！：）

Answer 3

如果使用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"}

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

还要尝试以下操作，看看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"}

请注意，'{“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，很不幸，postgres json并没有实现平等，但jsonb却实现了平等。因此，将json列迁移到jsonb上就可以了。

与Postgresql JSON数据列不同

4 个答案: