Below are the differences between the two parameters:
Max Rows to Retrieve: This will be the rows that will retrieved with each query. Note that it depends on how each client is implemented. For instance, PostgreSQL driver seems to honor this value and limits the data from DB, therefore both DB and DBX will be limited. However, other drivers may ignore this on the DB side (not like limiting the result by using LIMIT clause of SQL) and will honor it on DBX side only.
Fetch Size: This value will be set to the JDBC driver as a hint on how many rows to be fetched one time from the database. Word "hint" is emphasized, since this also highly depends on how the database drivers are implemented. For example, Oracle drivers seems to honor this setting and try to fetch the rows in chunk, so that it can avoid from out of client side (DBX) memory errors, while some drivers might not honor it and just send the whole result in one big bulk.
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