Notice: Undefined index: search_api_url in SearchApiDbService->indexItem() (line 332 of search_api_db\service.inc

I have the same problem with an undefined index about comment_count, instead.
The index is not being created, consequently...
Da.

I have the same problem. The problem for me seems to be related to including content type in the fields to index. After removing the content type from the index the error goes away and all items get index successfully.

Are you using the latest versions of all relevant modules (DB search, Search API, Entity API)? Did you try clearing all caches and re-saving the "Fields" form (maybe with changing some field back and forth to avoid the "No settings changed." note)?
And, can you reproduce this problem with Solr?

It's weird that this should occur for such various fields. It really seems like stale data in the server settings, which the above should fix.

I also just received this error which when URI was selected. Once URI was removed it worked fine.

I have all the latest versions of the relevant modules and this was just an index of two possible sets of data (first name , last name). Haven't tried solr yet.

Update - just deselected the URI field, indexed the search (no errors), then selected URI again and the indexing worked fine.

Please look at and test the patch in #1414138-10: Notice: Undefined index: search_api_access_node.

#2 saved my day.

After deploying a Feature containing Search Index settings to a (luckily) test server I had two problems:
* the search_api_db_default_node_index_type table was gone. This led to the error "Base table or view not found: 1146 Table 'MYDB.search_api_db_default_node_index_type'" every time I tried to clear or reindex.

The Feature included for the first time a Bundle Filter restricting the indexing to only one of our Content Types.
This was the only Data Alteration used.

I solved the missing index table by setting the Default Index to "No Server", saving, and then setting it to "DB" server again.
So much for problem 1.

Next problem was the problem with "Notice: Undefined index: XXX in SearchApiDbService->indexItem() "
In my case the indexed field XXX was a long time ago present on the main content type, but has since been removed.
It no longer appears in neither Fields or Facets so I cannot disable it.
I tried disabling the bundle filter, clearing and reindexing to no avail.
But the #2 comment about it having something to do with indexing the field Content Type saved my day!
After disabling just that Field I could index again. So thank you for that.
EDIT: it has nothing to do with the Content Type field - I realized it might be change of fields that triggered an initialisation of which fields should be indexed. And to test it, I restarted the process with a fresh db-dump, disabled another field and had the same result.
It actually makes more sense that way. Nothing special about the content type field.