a major change between bugzilla 4.0 and bugzilla 4.2 comes in the form of changes to searching. the searching implementation in 4.2 was rewritten from scratch, removing some seriously convoluted code and adding a large amount of test cases.
notable differences include:
search results limiting
search results are initially limited to 500 bugs, with an additional click required to show up to 10,000 bugs. it is no longer possible to generate a query which results in massive bug lists.
searching via APIs remains unlimited.
changes to relative dates
when using relative dates and times, -1w is now a synonym for -7d and means exactly 7 days.
previously, -1w meant the beginning of the week, which was confusing some users. the same confusion existed for -1d which was different from -24h, and for -1m which was different from -30d. now if you really want the beginning of the day, week or month, you must use -1ds, -1ws, and -1ms respectively, where "s" means "start of".
this change will affect existing saved searches using relative dates.
improved custom search builder
the custom search builder on the advanced search tab has been improved; it's now easier and quicker to construct queries.
complex queries built by the custom search interface are easier to build and have more sensible results, as they are built using a more intuitive logic.