Force vertical overlap in align

I can think of two ways, either use a negative space between the first and second row, or ignore the space of the sum. I prefer the second alternative.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
P(a|b) =  
\sum_{\substack{c, d, \\ e, f}}  \quad\quad 
      &P(a,b) \\[-1.9em]
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}

\begin{align}
P(a|b) =  
\smash[b]{\sum_{\substack{c, d, \\ e, f}}}  \quad\quad 
      &P(a,b) \\
\cdot &P(b,c) \\
\cdot &P(c,d) \\
\cdot &P(d,e) \\
\cdot &P(e,f)
\end{align}

\end{document}

Assuming you don't want to number each row, you can use an aligned subblock:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\begin{align}
  P(a|b) &=
  \sum_{\substack{c, d, \\ e, f}}
  \begin{aligned}[t]
    \quad &P(a,b) \\
    \cdot{} &P(b,c) \\
    \cdot{} &P(c,d) \\
    \cdot{} &P(d,e) \\
    \cdot{} &P(e,f)
  \end{aligned}\\
  &= x+y.
\end{align}
\end{document}

However, I would prefer to offset the following lines as follows:

\documentclass{article}

\usepackage{amsmath}

\begin{document}
\begin{align}
  P(a|b) &=
  \sum_{\substack{c, d, \\ e, f}}
  \begin{aligned}[t]
    &P(a,b) \\
    &\quad \cdot P(b,c) \\
    &\quad \cdot P(c,d) \\
    &\quad \cdot P(d,e) \\
    &\quad \cdot P(e,f)
  \end{aligned}\\
  &= x+y.
\end{align}
\end{document}