Creating an exposed filter for a field in an optional relationship effectively makes the relationship required.

If we have a table t1 with an optional relationship to table t2, views will generate SQL that looks something like:

SELECT [fields]
FROM t1
LEFT JOIN t2 on t1.nid = t2.nid

This has the intended effect of returning results from table t1 even if it doesn't have a match to table t2. Suppose, however, that table t2 has a field called "title" that we want to create an exposed filter for. When we submit the search with out anything in the form field (with the intention of not filtering that field at all) The SQL that views generates now looks like this:

SELECT [fields]
FROM t1
LEFT JOIN t2 on t1.nid = t2.nid
WHERE upper(t2.title) LIKE upper('%%')

The way that LEFT JOINs work in SQL is that they return NULL for all the fields in the left table if the join fails. Since NULL isn't LIKE any string, even one as accepting as '%%', the WHERE condition is not true for any rows in t1 which don't have a match in t2, and our optional relationship has effectively become required.

Views is too large for me to wrap my head around well enough to make sure that this solution is the right way to go about fixing the problem, but what I've done to fix this for myself is add an if() clause to all of the op_whatever() functions in handlers/views_handler_filter_string.inc that checks to see if $this->value is empty before adding the where clause. Thus:

  function op_contains($field, $upper) {
    $this->query->add_where($this->options['group'], "$upper($field) LIKE $upper('%%%s%%')", $this->value);
  }

becomes:

  function op_contains($field, $upper) {
    if(!empty($this->value)) {
      $this->query->add_where($this->options['group'], "$upper($field) LIKE $upper('%%%s%%')", $this->value);
    }
  }

Hopefully I've provided a thorough enough description of the problem and a potential solution for this to be an easy fix; I couldn't figure out a way to get this to work without modifying Views code directly, and Views gets updated often enough that maintaining it across updates will get old really fast.