Here are some recent papers on PCPs with small query complexity that I found interesting:
Here is a short list of authors that work on the connections between tree automata, logic and databases. For each author, I will just give one paper, but many more can be found on the respective web pages of these authors.
 Luc Segoufin
FO2(+1,<.~) on data trees, data tree automata and branching vector addition systems
 Nicole Schweikardt
A note ...