%!TEX root=./main.tex

\subsection{Relationship to Deterministic Query Runtimes}\label{sec:gen}
%We formalize our claim from \Cref{sec:intro} that a linear approximation algorithm for our problem implies that PDB queries (under bag semantics) can be answered (approximately) in the same runtime as deterministic queries under reasonable assumptions.
%Lastly, we generalize our result for expectation to other moments.

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%\revision{
%\subsection{Cost Model, Query Plans, and Runtime}
%As in the introduction, we could consider polynomials to be represented as an expression tree.
%However, they do not capture many of the compressed polynomial representations that we can get from query processing algorithms on bags, including the recent work on worst-case optimal join algorithms~\cite{ngo-survey,skew}, factorized databases~\cite{factorized-db}, and FAQ~\cite{DBLP:conf/pods/KhamisNR16}. Intuitively, the main reason is that an expression tree does not allow for `sharing' of intermediate results, which is crucial for these algorithms (and other query processing methods as well).
%}
%
%\label{sec:circuits}

%\mypar{The cost model}
%\label{sec:cost-model}
%So far our analysis of \Cref{prob:intro-stmt} has been in terms of the size of the lineage circuits.
%We now show that this model corresponds to the behavior of a deterministic database by proving that for any \raPlus query $\query$, we can construct a compressed circuit for $\poly$ and \bi $\pdb$ of size and runtime linear in that of a general class of query processing algorithms for the same query $\query$ on $\pdb$'s \dbbaseName $\dbbase$.  
% Note that by definition, there exists a linear relationship between input sizes $|\pxdb|$ and $|\dbbase|$ (i.e., $\exists c, \db \in \pxdb$ s.t. $\abs{\pxdb} \leq c \cdot \abs{\db})$).
% \footnote{This is a reasonable assumption because each block of a \bi represents entities with uncertain attributes.
% In practice there is often a limited number of alternatives for each block (e.g., which of five conflicting data sources to trust). Note that all \tis trivially fulfill this condition (i.e., $c = 1$).}
%That is for \bis that fulfill this restriction approximating the expectation of results of SPJU queries is only has a constant factor overhead over deterministic query processing (using one of the algorithms for which we prove the claim).
% with the same complexity as it would take to evaluate the query on a deterministic \emph{bag} database of the same size as the input PDB.
In~\Cref{sec:intro}, we introduced the structure $T_{det}\inparen{\cdot}$ to analyze the runtime complexity of~\Cref{prob:expect-mult}.
To decouple our results from specific join algorithms, we first abstract the cost of a join.

\begin{Definition}[Join Cost]
\label{def:join-cost}
Denote by $\jointime{R_1, \ldots, R_m}$ the runtime of an algorithm for computing the $m$-ary join $R_1 \bowtie \ldots \bowtie R_m$.
We require only that the algorithm must enumerate its output, i.e., that $\jointime{R_1, \ldots, R_m} \geq |R_1 \bowtie \ldots \bowtie R_m|$.
\end{Definition}

Worst-case optimal join algorithms~\cite{skew,ngo-survey} and query evaluation via factorized databases~\cite{factorized-db} (as well as work on FAQs~\cite{DBLP:conf/pods/KhamisNR16}) can be modeled as $\raPlus$ queries (though the query size is data dependent).
For these algorithms, $\jointime{R_1, \ldots, R_n}$ is linear in the {\em AGM bound}~\cite{AGM}.
% = |R_1| + \ldots + |R_n| + |R_1(\db) \bowtie \ldots \bowtie R_n(\db)|$.  
 Our cost model for general query evaluation follows from the join cost:

\noindent\resizebox{1\linewidth}{!}{
\begin{minipage}{1.0\linewidth}
  \begin{align*}
    \qruntimenoopt{R,\gentupset,\bound}                    & = |\gentupset.R| &
    \qruntimenoopt{\sigma \query, \gentupset,\bound}       & = \qruntimenoopt{\query,\gentupset} &
    \qruntimenoopt{\pi \query, \gentupset,\bound}          & = \qruntimenoopt{\query,\gentupset,\bound} + \abs{\query(\gentupset)}
  \end{align*}\\[-15mm]
  \begin{align*}
    \qruntimenoopt{\query \cup \query', \gentupset,\bound} & = \qruntimenoopt{\query, \gentupset,\bound} + 
                                            \qruntimenoopt{\query', \gentupset,\bound} +
                                            \abs{\query\inparen{\gentupset}}+\abs{\query'\inparen{\gentupset}} \\
    \qruntimenoopt{\query_1 \bowtie \ldots \bowtie \query_m, \gentupset,\bound} 
                                        & = \qruntimenoopt{\query_1, \gentupset,\bound} + \ldots + 
                                            \qruntimenoopt{\query_m,\gentupset,\bound} + 
                                            \jointime{\query_1(\gentupset), \ldots, \query_m(\gentupset)}
\end{align*}
\end{minipage}
}\\


Under this model, an $\raPlus$ query $\query$ evaluated over database $\gentupset$ has runtime $O(\qruntimenoopt{Q,\gentupset})$.
We assume that full table scans are used for every base relation access. We can model index scans by treating an index scan query $\sigma_\theta(R)$ as a base relation.
%Observe that 
% () .\footnote{This claim can be verified by e.g. simply looking at the {\em Generic-Join} algorithm in~\cite{skew} and {\em factorize} algorithm in~\cite{factorized-db}.} It can be verified that the above cost model on the corresponding $\raPlus$ join queries correctly captures the runtime of current best known .

Finally, \Cref{lem:circ-model-runtime} and \Cref{lem:tlc-is-the-same-as-det} show that for any $\raPlus$ query $\query$ and $\tupset$, there exists a circuit $\circuit^*$ such that $\timeOf{\abbrStepOne}(Q,\tupset,\circuit^*)$ and $|\circuit^*|$ are both $O(\qruntimenoopt{Q, \tupset,\bound})$. Recall we assumed these two bounds when we moved from \Cref{prob:big-o-joint-steps} to \Cref{prob:intro-stmt}.
%
%We now make a simple observation on the above cost model:
%\begin{proposition}
%\label{prop:queries-need-to-output-tuples}
%The runtime $\qruntimenoopt{Q}$ of any query $Q$ is at least $|Q|$
%\end{proposition}
%
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%
%We are now ready to formally state our claim with respect to \Cref{prob:intro-stmt}:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%\begin{Corollary}\label{cor:cost-model}
%  Given an $\raPlus$ query $\query$ over a \ti $\pdb$ with \dbbaseName $\dbbase$, we can compute a $(1\pm\eps)$-approximation of the expectation for each output tuple in $\query(\pdb)$ with probability at least $1-\delta$ in time
%
%  \[
%    O_k\left(\frac 1{\eps^2}\cdot\qruntimenoopt{Q,\dbbase}\cdot \log{\frac{1}{\conf}}\cdot \log(n)\right)
%    \]
%\end{Corollary}
%Atri: The above is no longer needed
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