Avoid unscope(:order) when limit_value is presented for count
#26972
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rafaelfranca
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Nov 5, 2016
If `limit_value` is presented, records fetching order is very important for performance. Should not unscope the order in the case.
kamipo
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I added a CHANGELOG entry! |
damianlegawiec
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Jun 25, 2017
`.distinct` with `.size` caused PostgreSQL errors introduced by rails/rails#26972
damianlegawiec
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Jun 25, 2017
`.distinct` with `.size` caused PostgreSQL errors introduced by rails/rails#26972
damianlegawiec
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Jun 25, 2017
`.distinct` with `.size` caused PostgreSQL errors introduced by rails/rails#26972
teohm
reviewed
Jul 19, 2017
| @@ -223,17 +223,17 @@ def operation_over_aggregate_column(column, operation, distinct) | |||
| end | |||
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| def execute_simple_calculation(operation, column_name, distinct) #:nodoc: | |||
| # PostgreSQL doesn't like ORDER BY when there are no GROUP BY | |||
| relation = unscope(:order) | |||
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@kamipo @rafaelfranca This change has caused an unexpected SQL error when a query contains both .distinct and .limit/.offset:
ActiveRecord::StatementInvalid: PG::InvalidColumnReference: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
In most SQL servers (e.g. PostSQL, SQL Server etc), columns listed in DISTINCT must also be listed in ORDER BY. This simple query that used to run in Rails 5.0 is now failing in Rails 5.1:
Model.distinct.order(updated_at: :desc).limit(10).count
because "count" operation injects column to DISTINCT list but keep the existing ORDER BY column list:
SELECT
COUNT(DISTINCT count_column)
FROM (
SELECT
DISTINCT "models"."id" AS count_column
FROM
"models"
ORDER BY
"models"."updated_at" DESC /* Error, column list not same as DISTINCT */
LIMIT 10) subquery_for_count
See issues reported by others: kaminari/kaminari#888, spark-solutions/spree@b5785ad
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Thanks for the report. I created the PR #29848 that fixed the issue.
kamipo
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Jul 21, 2017
Since rails#26972, `ORDER BY` is kept if `LIMIT` is presented for performance. But in most SQL servers (e.g. PostgreSQL, SQL Server, etc), `ORDER BY` expressions must appear in select list for `SELECT DISTINCT`. We should not replace existing select list in that case.
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If
limit_valueis presented, records fetching order is very importantfor performance. Should not unscope the order in the case.