The efficiency of a query execution plan depends on the accuracy of the selectivity estimates given to the query optimiser by the cost model. The cost model makes simplifying assumptions in order to
produce said estimates in a timely manner. These assumptions lead to selectivity estimation errors that have dramatic effects on the quality of the resulting query execution plans. A convenient assumption that is ubiquitous among current cost models is to assume that attributes are independent with each other. However, it ignores potential correlations which can have a huge negative impact on the accuracy of the cost model. In this paper we attempt to relax the attribute value independence assumption without unreasonably deteriorating the accuracy of the cost model. We propose a novel approach based on a particular type of Bayesian networks called Chow-Liu trees to approximate the distribution of attribute values inside each relation of a database. Our results on
the TPC-DS benchmark show that our method is an order of magnitude.
more precise than other approaches whilst remaining reasonably efficient
in terms of time and space.
In this experiment I looked into a way fo measuring the ground bounce generated by capacitively loading 5 outputs of a Texas Instrument small outline package Hex-Flip Flop chip and discharging them all simultaneously.
The parameters of a fuel-optimal mission from Kerbin via Eve to an outer planet with an arbitrary orbit radius were calculated. The $\Delta v$ gain of performing the Eve flyby instead a direct Hohmann transfer to the outer planet is on the order of 50 m/s. Eccentricity and inclination of planets' orbits were not considered.
Watermarking technique for the image is an efficient method for protecting copyright image, and also a huge topic in cryptography. In this paper, two spread spectrum watermarking scheme, the Convolution Image-based Model (CIM) and the Exponential Convolution Image-based Model (ECIM) are going to be formulated and discussed. The watermarking experiment result will be shown and discussed, focusing on the attack scheme, protectability, and the information encryption of the watermark. We will show that the convolution image-based model for invisible watermark is weak of protectability, but it is able to hide the information (the size of watermark must be less than the original image) and store inside the image.