Fix for 823431: pt-query-advisor hangs on big queries

This was caused by a regex backtracking itself to a halt. The solution
was to add a positive lookahead that searches for the floating but mandatory
substrings that the regex needs; if those exist, then eventually it will
match, but if they don't, it'll fail quickly.

lib/SQLParser.pm

@@ -371,6 +371,7 @@ sub parse_insert {
 
    # Parse INTO clause.  Literal "INTO" is optional.
    if ( my @into = ($query =~ m/
+            (?=VALUE|SE(?:T|LECT))  # Avoid a backtracking explosion
             (?:INTO\s+)?            # INTO, optional
             (.+?)\s+                # table ref
             (\([^\)]+\)\s+)?        # column list, optional