The sample Sphinx search configuration file in ./docs/sphinx.sample.conf contains 2 errors which result in indexer only updating the delta index with the most recent post and not every post since the last indexing of the main index.
The first error is at line 44:
As per the Sphinx documentation, character encoding must be reset for the delta index configuration, so line 44 should instead read:
The second error is at line 64:
By not specifying a new blank sql_query_post_index value, it repeats the last sql_query_post_index definition for the main index, which sets the SPHINX_TABLE max_doc_id column to the most recent post ID. Every time the delta indexer is run, max_doc_id is set to the most recent post_id in the database and so the delta index will only ever contain the most recent forum post.
Line 64 should therefore be amended to:
These amendments are also consistent with the pro forma sphinx.conf file generated by the ACP Sphinx search module.