Introduce 'relation' arg to meta_query

[scribu]

Just like tax_query has an optional 'relation' arg that allows 'OR' relationships between queries, so should meta_query.

Thats my try!

What's the deal with $distinct = 'DISTINCT'; ?

Also, it's cleaner to set $relation this way:

	if ( isset( $meta_query['relation'] ) && strtoupper( $meta_query['relation'] ) == 'OR' ) {
		$relation = 'OR';
	} else {
		$relation = 'AND';
	}

If DISTINCT isn't used the result includes same posts again and again(because of inner joins)

Steps to reporduce:

• Create a post with two meta keys 'foo' and 'bar'
• Create a post with meta key 'foo'
• Create a post with meta key 'bar'
• Run this query

  $q = new WP_Query(
  array(
  	'fields' => 'ids',
  	'ignore_sticky_posts' => true,
  	'meta_query' => array(
  		'relation' => 'OR',
  		array(
  			'key' => 'foo',
  			'value' => array( 'foobar' ),
  			'compare' => 'IN'
  		),
  		array(
  			'key' => 'bar',
  			'value' => array( 'foobar2' ),
  			'compare' => 'IN'
  		)
  	)
  ) );
  print_r( $q->posts );
  

Also, it's cleaner to set $relation this way:

Agreed.

If DISTINCT isn't used the result includes same posts again and again(because of inner joins)

Ok, but doesn't this happen without the patch as well?

Replying to scribu:

If DISTINCT isn't used the result includes same posts again and again(because of inner joins)

Ok, but doesn't this happen without the patch as well? i.e. with the default AND relation.

IIRC only OR'ed queries with inner joins cause that problem.

Ah, right, because the same post can have both fields.

Related: #17165

Related: #15292

Ah, but the DISTINCT is still unnecessary, since there's a GROUP BY ID clause automatically added on any taxonomy or meta query.

Replying to scribu:

Ah, but the DISTINCT is still unnecessary, since there's a GROUP BY ID clause automatically added on any taxonomy or meta query.

Yes, but currently GROUP BY is set only if meta_key is set, if queried using meta_query there is no GROUP BY and hence DISTINCT is needed. However, DISTINCT is unnecessary once #17165 goes in and the patch will need refresh.

(In [17699]) Introduce WP_Meta_Query and relation support. Props scribu, greuben. fixes #17165 #17011