I am looking at the documentation for the expire field in the cache tables and the documentation in CacheBackendInterface.php says:

  * @param int $expire
   *   One of the following values:
   *   - CacheBackendInterface::CACHE_PERMANENT: Indicates that the item should
   *     not be removed unless it is deleted explicitly.
   *   - A Unix timestamp: Indicates that the item will be considered invalid
   *     after this time, i.e. it will not be returned by get() unless
   *     $allow_invalid has been set to TRUE. When the item has expired, it may
   *     be permanently deleted by the garbage collector at any time.
   * @param array $tags

But when I look in a cache table, for example, cache_render I see many of the entires have NULL in the expire column and I noticed it only get clear on a complete cache clear drush cr

I am looking at the documentation for the expire field in the cache tables and the documentation in CacheBackendInterface.php says:

@param int $expire
One of the following values:

• CacheBackendInterface::CACHE_PERMANENT: Indicates that the item should not be removed unless it is deleted explicitly.
• A Unix timestamp: Indicates that the item will be considered invalid after this time, i.e. it will not be returned by get() unless $allow_invalid has been set to TRUE. When the item has expired, it may be permanently deleted by the garbage collector at any time.

When I look in a cache table, for example cache_render, I see that many entries have NULL in the expire column and I noticed it only get clear on a complete cache clear, such as after running drush cr.

What does a null value set for the expire column in a cache table mean?