make "Query Drop Down (Single Select)" searchable

My Query Drop Down (Single Select) has over 10.000 entries. Therefore I need a "type ahead" or "instant search" functionality on that field. What is the best way? Could I use selectize?

You can have a text field displayed before it then in your query used in your query select substitute in the text field to filter by it. Next use the Update On feature provided by CB Core Fields Ajax so your query select field refreshes itself when your text field changes.

Thanks. But that means the user has to first type in one field, then click on another field to see if the amount of typed letters decreased the amount of options far enough and if not click in the text field again back to the keyboard and type again, then click in the drop down again to see how many items are now in the list if still too many click in the text field again keep typing ... that is a UI from the 80ies. Is there no JS solution like selectize? Surely I am not the first one facing this problem.

No, there is no JS solution. You need ajax endpoints for a JS solution. The UI can be improved by using CSS and moving things around. Best I can suggest is use a Text field with the new Auto Complete feature provided by CB Code Field and CB Query Field plugins as shown below.

www.joomlapolis.com/blog/kyle/18831-cb-code-field-2-0-0

This maybe expanded in the future to work with select2 JS dropdowns to act as an endpoint for select2 search results.

Thanks, got it working with the additional text field. Some problems remain:

1. How do I make sure the query for the select field does not run on page load but only on text field change?
2. How do I make sure the select field only updates when at least 3 chars are entered in the text field?
3. How do I make sure the select field contains max. 10 entries?

You're able to have what you're wanting now via the Autocomplete feature. Both CB Query Field and CB Code Field provide Autocomplete now for text fields. This would still allow free form editing of the field though since it's a text field.

1. How do I make sure the query for the select field does not run on page load but only on text field change?

Ensure your query has the appropriate filters in it to limit its result set when no text field value exists. You could even surround the query with an IF substitution checking if the text field has a value so it won't query unless text field value is supplied for example.

2. How do I make sure the select field only updates when at least 3 chars are entered in the text field?

I assume you mean with the Update On functionality provided by CB Core Fields Ajax; you can't. There's no feature to limit length. You could add such a limitation to your query field I suppose again using an IF substitution, but with a regexp operator to check if it has at least 3 characters.

3. How do I make sure the select field contains max. 10 entries?/quote]
Add a LIMIT statement to your SQL in your query select.