XAR export executes many database queries on large wikis even when exporting just a single page

Steps to reproduce:

1. Have a lot of spaces nested inside a common root space (like, 200k spaces).
2. Perform a XAR export of a single page that is also in that root space.

Expected result:

The export is fast and when checking a trace, e.g., on Glowroot, only a few export-related database queries are shown.

Actual result:

The export is slow and checking database queries shows 200k queries of

SELECT DISTINCT xwikidocum0_.XWD_NAME AS col_0_0_
    FROM xwikidoc xwikidocum0_
   WHERE xwikidocum0_.XWD_WEB = ?
ORDER BY xwikidocum0_.XWD_NAME ASC