paper-BagRelationalPDBsAreHard/prob-def.tex

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\subsection{Problem Definition}\label{sec:expression-trees}

We first formally define circuits, an encoding of polynomials that we use throughout the paper.  Since we are particularly using \emph{lineage} circuits, we drop the term lineage and only refer to them as circuits.

For illustrative purposes consider the polynomial $\poly(\vct{X}) = 2X^2 + 3XY - 2Y^2$ over $\vct{X} = [X, Y]$.

We represent query polynomials via {\em arithmetic circuits}~\cite{arith-complexity}, a standard way to represent polynomials over fields (particularly in the field of algebraic complexity) that we use for polynomials over $\mathbb N$ in the obvious way.

\begin{Definition}[Circuit]\label{def:circuit}
A circuit $\circuit$ is a Directed Acyclic Graph (DAG) whose source nodes (in degree of $0$) consist of elements in either $\reals$ or $\vct{X}$.  The internal nodes and sink node of $\circuit$ have binary input and are either sum ($\circplus$) or product ($\circmult$) gates.  

$\circuit$ additionally has the following members: \type, \val, \vari{partial}, \vari{input}, \degval and \vari{Lweight}, \vari{Rweight}, where \type is the type of value stored in the node $\circuit$ (i.e. one of $\{\circplus, \circmult, \var, \tnum\}$, \val is the value stored (a constant or variable), and \vari{input} is the list of \circuit 's inputs where $\circuit_\linput$ is the left input and $\circuit_\rinput$ the right input.  The member \degval holds the degree of \circuit.  When the underlying DAG is a tree (with edges pointing towards the root), we will refer to the structure as an expression tree \etree.  Note that in such a case, the root of \etree is analogous to the sink of the \circuit.
\end{Definition}

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As stated in ~\Cref{def:circuit}, every internal node has at most two in-edges, is labeled as an addition or a multiplication node, and has no limit on its outdegree.
Note that if we limit the outdegree to one, then we get expression trees. 
\begin{Example}
The circuit \circuit in ~\Cref{fig:circuit-express-tree} encodes the polynomial $XY + WZ$.  Note that such an encoding lends itself naturally to having all gates with an outdegree of $1$.  Note further that \circuit is indeed a tree with edges pointing towards the root.
\end{Example}

\begin{figure}[t]
	\begin{subfigure}[b]{0.45\linewidth}
		\centering
		\begin{tikzpicture}[thick]
			\node[tree_node] (a1) at (0, 0){$\boldsymbol{X}$};
			\node[tree_node] (b1) at (1, 0){$\boldsymbol{Y}$};
			\node[tree_node] (c1) at (2, 0){$\boldsymbol{W}$};
			\node[tree_node] (d1) at (3, 0){$\boldsymbol{Z}$};
	
			\node[tree_node] (a2) at (0.5, 1){$\boldsymbol{\circmult}$};
			\node[tree_node] (b2) at (2.5, 1){$\boldsymbol{\circmult}$};
	
			\node[tree_node] (a3) at (1.5, 2){$\boldsymbol{\circplus}$};
	
			\draw[->] (a1) -- (a2);
			\draw[->] (b1) -- (a2);
			\draw[->] (c1) -- (b2);
			\draw[->] (d1) -- (b2);
			\draw[->] (a2) -- (a3);
			\draw[->] (b2) -- (a3);
			\draw[->] (a3) -- (1.5, 2.5);
		\end{tikzpicture}
		\caption{Circuit encoding $XY + WZ$, a special case of an expression tree}
		\label{fig:circuit-express-tree}
	\end{subfigure}
	\hspace{5mm}
	\begin{subfigure}[b]{0.45\linewidth}
		\centering
		\begin{tikzpicture}[thick]
			\node[tree_node] (a1) at (0, 0) {$\boldsymbol{X}$};
			\node[tree_node] (b1) at (1.5, 0) {$\boldsymbol{2}$};
			\node[tree_node] (c1) at (3, 0) {$\boldsymbol{Y}$};
			\node[tree_node] (d1) at (4.5, 0) {$\boldsymbol{-1}$};
	
			\node[tree_node] (a2) at (0.75, 0.75) {$\boldsymbol{\circmult}$};
			\node[tree_node] (b2) at (2.25, 0.75) {$\boldsymbol{\circmult}$};
			\node[tree_node] (c2) at (3.75, 0.75) {$\boldsymbol{\circmult}$};
	
			\node[tree_node] (a3) at (0.55, 1.5) {$\boldsymbol{\circplus}$};
			\node[tree_node] (b3) at (3.75, 1.5) {$\boldsymbol{\circplus}$};
	
			\node[tree_node] (a4) at (2.25, 2.25) {$\boldsymbol{\circmult}$};		
	
			\draw[->] (a1) -- (a2);
			\draw[->, thick] (a1) -- (a3);
			\draw[->] (b1) -- (a2);
			\draw[->] (b1) -- (b2);
			\draw[->] (c1) -- (c2);
			\draw[->] (c1) -- (b2);
			\draw[->] (d1) -- (c2);
			\draw[->] (a2) -- (b3);
			\draw[->] (b2) -- (a3);
			\draw[->] (c2) -- (b3);
			\draw[->] (a3) -- (a4);
			\draw[->] (b3) -- (a4);
			\draw[->] (a4) -- (2.25, 2.75);
		\end{tikzpicture}
		\caption{Circuit encoding of $(X + 2Y)(2X - Y)$}
		\label{fig:circuit}
	\end{subfigure}
	\caption{ }
\end{figure}

We ignore the remaining fields (\vari{partial}, \vari{Lweight}, and \vari{Rweight}) until \Cref{sec:algo}.


The semantics of circuits follows the obvious interpretation.  We next define its realtionship with polynomials formally:
\begin{Definition}[$\polyf(\cdot)$]\label{def:poly-func}
Denote $\polyf(\circuit)$ to be the function from circuit $\circuit$ to its corresponding polynomial.  $\polyf(\cdot)$ is recursively defined on $\circuit$ as follows, with addition and multiplication following the standard interpretation for polynomials:
\begin{equation*}
	\polyf(\circuit) = \begin{cases}
					\polyf(\circuit_\lchild) + \polyf(\circuit_\rchild)			&\text{ if \circuit.\type } = \circplus\\
					\polyf(\circuit_\lchild) \cdot \polyf(\circuit_\rchild)		&\text{ if \circuit.\type } = \circmult\\
					\circuit.\val									&\text{ if \circuit.\type } = \var \text{ OR } \tnum.
				\end{cases}
\end{equation*}
\end{Definition}

Note that $\circuit$ need not encode an expression in standard monomial basis, while as stated previously a polynomial is considered to be in SMB, and the output of \polyf($\cdot$) is therefore in SMB.  For instance, $\circuit$ could represent a compressed form of the running example, such as $(X + 2Y)(2X - Y)$
, as shown in \Cref{fig:circuit}.

\begin{Definition}[Circuit Set]\label{def:circuit-set}
$\circuitset{\smb}$ is the set of all possible circuits $\circuit$ such that $\polyf(\circuit) = \poly(\vct{X})$.
\end{Definition}

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The circuit of \Cref{fig:circuit} is an element of $\circuitset{\smb}$.  One can think of $\circuitset{\smb}$ as the infinite set of circuits each of which model an encoding (factorization) equal to $\polyf(\circuit)$.   
%\supset \{2X^2 + 3XY - 2Y^2, (X + 2Y)(2X - Y), X(2X - Y) + 2Y(2X - Y), 2X(X + 2Y) - Y(X + 2Y)\}$.  
Note that ~\Cref{def:circuit-set} implies that $\circuit \in \circuitset{\polyf(\circuit)}$.

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\medskip

\noindent We are now ready to formally state our \textbf{main problem}.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{Definition}[The Expected Result Multiplicity Problem]\label{def:the-expected-multipl}
Let $\vct{X} = (X_1, \ldots, X_n)$, and $\pdb$ be an $\semNX$-PDB over $\vct{X}$ with probability distribution $\pd$ over assignments $\vct{X}  \to [0,1]$, $\query$ an n-ary query, and $t$ an n-ary tuple.
  The \expectProblem is defined as follows:
  
\hspace*{5mm}\textbf{Input}: A circuit $\circuit \in \circuitset{\smb}$ for $\poly(\vct{X}) = \query(\pxdb)(t)$\hspace*{5mm}\textbf{Output}: $\expct_{\vct{W} \sim \pd}[\poly(\vct{W})]$
\end{Definition}
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