Improved performance of smart completions

[tkopets]

On lengthy SQL statements with lots of identifiers and aliases the performance was poor, this commit improves performance of smart completions for most widely used case - when prefix does not contain any dots (no parent object specified).

[tkopets]

@mtxr Those codacy checks seem to be stuck.

[mtxr]

I took a look there. Seems to be ok. Probably is just an instability.

Approving as administrator.

Thanks!

[tkopets]

Thanks man!