The query gives the data needed to populate fields of the Solr document in full-import

The deltaImportQuery gives the data needed to populate fields when running a delta-import

The deltaQuery gives the primary keys of the current entity which have changes since the last index time

The parentDeltaQuery uses the changed rows of the current table (fetched with deltaQuery) to give the changed rows in the parent table. This is necessary because whenever a row in the child table changes, we need to re-generate the document which has that field.