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join_buffer_size = 32M
read_rd_buffer_size = 32M
innodb_buffer_pool_size = 512M1536M
innodb_file_per_table = 1
innodb_file_format = barracuda

Reading the documentation on MySQL, there is in 5.6 a feature called "batched key access", which on the surface should help with large joins. However, when tested on this query, it appeared to make no difference - there was no change in either execution plan (it would say Using join buffer (Batched Key Access)) or in the execution time. But there may be other queries that we execute where it could come into play. See https://dev.mysql.com/doc/refman/5.6/en/bnl-bka-optimization.html

Even with a large query with OPTIONALs (e.g. the listview-authorInAuthorship select), MySQL can't be provoked into using batched key access on a join. It does however start doing batched nested loops. This isn't hideous - under 4 seconds on a 1700+ publication profile - but not quite as fast as Virtuoso (2 seconds).

Despite the SDB layout using hashes for keys, and all joins based on equivalence, the architecture of MySQL is quite heavily leveraged on BTREE indexes, rather than hash indexes. MariaDB (a MySQL fork) adds batched hash joins to those offered by MySQL - it may be that this could be leveraged for better performance.