[quote=Buckeye]
My tst files where opening in the existing instance but my sql files where not. I looked in the Windows registry and changed:
HKEY_CLASSES_ROOT\.sql data to PL/SQL Developer
Now the sql files open in the existing instance! I first looked at the .tst entry to see what it looked like.
[/quote]
This is my .tst setting in the registry:
[HKEY_CLASSES_ROOT\.tst]
@="PL/SQL Developer 14"

And this is my .pck setting:
[HKEY_CLASSES_ROOT\.pck]
@="PL/SQL Developer 14"

The first opens in an existing instance, the second always opens a new instance. So I guess is IS something in PL/SQL developer...