title: Spanner how 'deep' is primary key character count indexed? e.g. would there be mega-hot spot if the first 2 characters are the same +1
body: <p>I'm using Spanner for the first time.</p>
  <p>I am using a UUIDv4 for a primary key, but I'm building a table that I think from my understanding could benefit from another primary key (or maybe a secondary index is better? but i'm still learning how to use those in Spanner and my knowledge there is less, especially how to build / maintain after new or modified rows).</p>
  <p>Because I will be in some instances selecting by WHERE primary_key AND another_primary_phone_number_key</p>
  <p>For instance this table let's call it users would have org_id | uuid | formatted_phone_number, where all phone numbers start with +1 before getting to the mostly even distributed 1-9 3rd string character</p>
  <p>Here is how that schema would look - shortened for the example</p>
  <p>Would this actually distribute evenly in I believe Spanner calls them Splits? Or would it be one giant hotspot because they all start with +?</p>
  <pre><code>CREATE TABLE users (
    org_id STRING(MAX) NOT NULL,
    user_uuid STRING(MAX) NOT NULL,
    formatted_phone_number STRING(MAX),
    fb_uuid STRING(MAX),
    FOREIGN KEY(org_id) REFERENCES accounts(org_id),
  ) PRIMARY KEY(org_id, user_uuid, formatted_phone_number),
    INTERLEAVE IN PARENT accounts ON DELETE NO ACTION;
  </code></pre>