Many-to-one filters, OR-groups and INNER JOIN

Problem/Motivation

TL;DR: Fields using the "Many to one" helper filter are considered as "required", even when they are in an OR group which should make it optional, due to a somewhat too aggressive optimization.

Steps to reproduce the problem:

• create a content type, named Issue, with one "list" field: let's say a field named "Version", of type List (integer), with allowed values 1, 2 and 3
• create a second type, named "Article", without any custom field
• create one Issue node with version 1 and one Article node
• create a view using the content as base table
• add a filter on the Version list field, choose "Is one of", pick the option "1"
• add a filter on Content: Type restricting it to content type 'issue'
• add a filter on Content: Type 'article'
• re-arrange the filter to put them into two groups, like so:
  

  group 1 (AND each filter)
  Content type: Article
  
  OR
  
  group 2 (AND each filter)
  Content type: Issue
  Version: 1
  

The expected result would be to get all the articles and all the issues affecting version "1".

The actual result though right now is that you only get issues nodes.

Looking at the query, the join clauses for the version field is an INNER JOIN, which explains why all the article nodes are disappering. This is coming from views_many_to_one_helper::ensure_my_table and views_many_to_one_helper::summary_join, which are trying to use INNER JOINs to get a speed benefit "when we know we can".

Proposed resolution

Treat the cases where the filter is in an OR-group, or where all the filter groups are OR-ed together as unsafe for INNER JOIN, falling back to LEFT JOIN.

Remaining tasks

Review, write tests.

User interface changes

None.

API changes

None.