I currently have a database that stores some transactions for several users, for example purchasing or selling products is stored as 1 entry in the transactions table. It can also store profits made between transactions (stored in another table called profits).  I also need to store a daily history of purchases, revenues and profits for each user. I have a history table that looks something like this:  user_iduser   date        total_buy  total_sell total_profit     2500    2016-04-02     24555       45121        2566   Right now, the way I keep the history table updated is outside the database side. The application automatically calculates and updates this table everytime the user logs in (after fetching whatever new transactions come from that day) and updates the corresponding totals for that day. The application also runs a script everyday at midnight that fetches every user's transactions and updates their totals for the previous day. However, I would prefer if this behavior was done by the database instead if possible (the less code the better).  So my question is: would creating triggers be a good way to do this, performance wise? It would be simple to create a trigger that, for example updates a user's history.total_buy column everytime a new 'buy' transaction is inserted, but considering I have several million transactions already stored and thousands of new ones arriving a day, wouldn't the constant trigger calls put a stress on the database? Or am I just getting worried for no reason?
A post here on DBA.StackExchange () has spawned an interesting question (at least, interesting to me) regarding performance in MySQL.  The context is that we want to insert a record in a table for each row that is updated.  Before the row is updated, we want to store off a previous value and then increment one of the columns (a "version" column).  If we do this inside a trigger, it works out nicely.  For MySQL, the triggers are , so it would be an easy solution.  Select the data currently in the table, insert it into the logging table, and the update the "version" column in the new data.  However, it's possible to move this logic to a stored procedure. If you do that, you're performing the insert, then incrementing the "version" column in the table.  The entire thing would be set based.   So, when it comes to performing this insert, would this be more performant to use the set-based stored procedure approach or a trigger based approach?  This question is for MySQL (since it has row-by-row triggers), although it could apply to other row-by-row trigger DBMSs.
This is what happens:  sudo apt-get install vlc Reading package lists... Done Building dependency tree        Reading state information... Done E: Unable to locate package vlc   Please help. I would also like to know when this error occurs, I've seen it before too. "#include" a text file in a C program as a char[] Does borrowing from my 401(k) make sense in my specific circumstance? Do "Best by: XX-XX-XX" foods have an expiration date? How long will foods that have "Best by: XX-XX-XX" on the product last past that date?  I have tried looking online, and all I can find is anecdotal evidence, which isn't very assuring for me since there seems to be no specific "x food will expire y weeks after its best by date".  Is there a rule of thumb or some guide out there that can tell me how long a "best by" food will last? Or is it safe to assume that they never expire, since it's not actually labelled on the product? How to add appendix in thesis using LyX I want to add an appendix (questionnaire etc.) at the end of my thesis after the references. I use the article class and work with LyX: Document -&gt; Start appendix here. But I am not getting it in the TOC.  \documentclass[12pt,english]{article} \usepackage[T1]{fontenc} \usepackage[latin9]{inputenc} \setlength{\parskip}{\smallskipamount} \setlength{\parindent}{0pt} \usepackage{color} \definecolor{document_fontcolor}{rgb}{0, 0, 0} \color{document_fontcolor} \usepackage{babel} \usepackage{float} \usepackage{textcomp} \usepackage{pdfpages} \usepackage{tipa} \usepackage{amsmath} \usepackage{graphicx} \usepackage{setspace} \PassOptionsToPackage{normalem}{ulem} \usepackage{ulem} \usepackage[unicode=true,  bookmarks=true,bookmarksnumbered=false,bookmarksopen=false,  breaklinks=false,pdfborder={0 0 0},backref=false,colorlinks=false]  {hyperref}  \makeatletter  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands. %% Because html converters don't know tabularnewline \providecommand{\tabularnewline}{\\} %% A simple dot to overcome graphicx limitations \newcommand{\lyxdot}{.}   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Textclass specific LaTeX commands. \numberwithin{equation}{section}  \@ifundefined{date}{}{\date{}} \@ifundefined{showcaptionsetup}{}{%  \PassOptionsToPackage{caption=false}{subfig}} \usepackage{subfig} \makeatother  \appendix \noindent \begin{center} \textbf{Questionnaire (Case Study 1)} \par\end{center} What is the $n$ times iterate of $f(x)=\frac{x}{\sqrt{1+x^2}}$? How do I upgrade to a newer version of Ubuntu? What are the different ways I can use to upgrade Ubuntu from one release to another? I keep getting an error in line 38 saying "Extra alignment tab has been changed to \cr." \documentclass [12pt]{article} \oddsidemargin=0pt \topmargin=0pt \textwidth=6.5in \textheight=8.5in \begin{document} \begin{center} {\large\bf\LaTeX\ Example 6} \end{center} We can typeset array structures \begin{equation} \mathbf{A}\times \mathbf{B}=\left| \begin{array}{ccc} \mathbf{i} &amp; \mathbf{j}  &amp; \mathbf{k} \\      a_1  &amp;      a_2        &amp; a_3 \\       b_1  &amp;      b_2        &amp; b_3  \end{array}  \right| \end{equation} \begin{quote} \begin{verbatim} \begin{equation} \mathbf{A}\times \mathbf{B}=\left| \begin{array}{ccc} \mathbf{i} &amp; \mathbf{j}  &amp; \mathbf{k} \\  a_1  &amp;  a_2   &amp; a_3 \\  b_1  &amp;  b_2   &amp; b_3 \end{array} \right| \end{equation} \end{verbatim} \end{quote} Or \begin{equation} \mathbf{A}=\left( \begin{array}{ccc} a_{11} &amp; a_{12} &amp; \cdots &amp; a_{1n} \\ a_{21} &amp; a_{22} &amp; \cdots &amp;  a_{2n} \\ \vdots &amp; \vdots &amp; \ddots &amp; \vdots \\ a_{n1} &amp; a_{n2} &amp; \cdots &amp; a_{nn} \end{array} \right) \end{equation} \end{document} Model selection with low N? I've got a study which is kinda messed up by the design...Turns out I ended up with about 50 patients of which 80% have the outcome and 20% don't (binary outcome).  I've been turning in my bed for the last month trying to figure out what to do with this. The only real answer is "don't" but I have to deliver something in the next days as it's part of a master's thesis.  I have about 10 predictors of interest for the outcome (various variable types) and the research question is whether any of these predictors can predict the outcome. Some already have an established correlation in the literature while some are original hypothesis. Several are significant with a univariate logistic regression, but it doesn't look good in any other way than the actual p-value.  Now obviously I'm not going to be able to answer this research question sufficiently but if you were in my shoes, what sort of statistical analysis would you perform to relay to your supervisors?  EDIT: Link to my (anonymized) data (CSV): How to calculate: $\sum_{n=1}^{\infty} n a^n$ I've tried to calculate this sum:  $$\sum_{n=1}^{\infty} n a^n$$  The point of this is to try to work out the "mean" term in an exponentially decaying average.  I've done the following:  $$\text{let }x = \sum_{n=1}^{\infty} n a^n$$ $$x = a + a \sum_{n=1}^{\infty} (n+1) a^n$$ $$x = a + a (\sum_{n=1}^{\infty} n a^n + \sum_{n=1}^{\infty} a^n)$$ $$x = a + a (x + \sum_{n=1}^{\infty} a^n)$$ $$x = a + ax + a\sum_{n=1}^{\infty} a^n$$ $$(1-a)x = a + a\sum_{n=1}^{\infty} a^n$$  Lets try to work out the $\sum_{n=1}^{\infty} a^n$ part:  $$let y = \sum_{n=1}^{\infty} a^n$$ $$y = a + a \sum_{n=1}^{\infty} a^n$$ $$y = a + ay$$ $$y - ay = a$$ $$y(1-a) = a$$ $$y = a/(1-a)$$  Substitute y back in:  $$(1-a)x = a + a*(a/(1-a))$$ $$(1-a)^2 x = a(1-a) + a^2$$ $$(1-a)^2 x = a - a^2 + a^2$$ $$(1-a)^2 x = a$$ $$x = a/(1-a)^2$$  Is this right, and if so is there a shorter way?  Edit:  To actually calculate the "mean" term of a exponential moving average we need to keep in mind that terms are weighted at the level of $(1-a)$. i.e. for $a=1$ there is no decay, for $a=0$ only the most recent term counts.  So the above result we need to multiply by $(1-a)$ to get the result:  Exponential moving average "mean term" = $a/(1-a)$  This gives the results, for $a=0$, the mean term is the "0th term" (none other are used) whereas for $a=0.5$ the mean term is the "1st term" (i.e. after the current term). Java, pointer like way to program this How do I rotate a tikz rectangle about a specific axis? I am trying to create a beamer animation using tikz, but I can't even get basic rotating of a tikz image working as desired.  I have a rectangle specified by  \draw[rounded corners,dotted] (-3.5,0) rectangle (-0.5,2);   I would like to rotate this rectangle about the point (-1,.5) (which is near the bottom right corner of the rectangle).  I've tried:  {\rotatebox[origin=c,x=-1,y=.5]{130}{             \draw[rounded corners,dotted] (-3.5,0) rectangle (-0.5,2);}}   But this produces an error.  If I just use the origin=c option, it compiles, but it rotates the rectangle about the point (0,0).  I've also tried it with just the x=-1,y=.5 (without the origin option), and it fails to compile.  I have searched the documentation, which is where I learned about the options x=xdim and y=ydim.  However, I can't find a single example demonstrating how these options should be used.  Any suggestions would be greatly appreciated.    Also, if there is a better command for rotating things that will be more suitable for use in a beamer animation, please let me know.  Thanks! Verb form of "must needs" Then touching the shoulder of a townsman who stood near to him, he addressed him in a formal and courteous manner:      "I pray you, good Sir," said he, "who is this woman? -- and wherefore is she here set up to public shame?"      "You must needs be a stranger in this region, friend," answered the townsman, looking curiously at the questioner and his savage companion, "else you would surely have heard of Mistress Hester Prynne and her evil doings. She hath raised a great scandal, I promise you, in godly Master Dimmesdale's church. "   As far as I know we always use first form of the verb after "must". For instance: "You must go". We never say "You must goes". Also, we never use -s/es with a verb that comes with "You". What are the circumstances in which writer has used "needs" after "You must" in "You must needs to be a stranger in this region."? Can black holes actually merge? If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a dipole field? Inequality vs. Inequation, Equation vs. Equality When talking about statements that two terms are mathematically (un-)equal (e.g., 1 = 1, 1 &lt; 2, or 1 != 2), what is the correct notion for such mathematical statements? Are there special cases w.r.t. "mixed" relations like x &lt;= y? Can some notions be used interchangeably?  It seems many online sources use "equation" and "inequality" but there is also a . Apart from personal posts in forums, I have not found any sources using "equality" in such a situation, yet. Systematically, I would think that "equation" and "inequation" would make more sense than "equation" and "inequality", but not everything (probably very few things) in natural languages have developed according to a purely logical system.  Note that this question is more general than  for that it particularly asks for the relation between the notion for equations and the one for inequalities. Equations are not part of the existing question at all. Error: j_id0:j_id1:j_id28:j_id30: Validation Error: Value is required What are the pros and cons of adding a vapor barrier behind shower backer board? After much research, I have a lot of conflicting advise about the need to add a vapor barrier before installing the backer (hardy / cement) board for my bathroom shower. The &quot;expert&quot; at Home Depot, and  seem to think this is a very good idea, and even suggest sealing it with roofing tape.  (The guy on Youtube tapes it directly to the sill of tub.) However, when I asked about the tape and how far down I should go at Lowes, they told me they'd never hear of anyone doing that, and that it didn't seem like a good idea.   Likewise,  putting down caulking on the sill before applying the backer board, and then again after before adding tile, and then caulking a 3rd time under the tiles before grouting. Should I add a vapor barrier between the frame and the cement board?  Why or why not? Calculus book recommendations (for complete beginner) Well I have not started calculus yet but I am really keen to. I would love if you suggest some books.  Points to be noted:   I really don't like the way textbooks are written so please no "textbooks"  I am COMPLETELY beginner in calculus. I know a little bit of trigonometry and what functions are but not really in depth.The book should start from the base, I mean really from the base. I may require a precalculus book too if the books don't cover that. I would really love if the book shows how calculus was developed, why it was developed and things like that.   Well,thanks in advance! Generalizing a sequence. Consider a sequence of the form:  $x[n]=-1,-1...,1,1,...,-1,-1...,1,1... \quad n&gt;0$  One can think of this as a square wave ($\pm1$) with a 50 % duty cycle (coming from EE).  For the simplest case of such a series i.e. $-1,+1,-1,+1,-1...$, a general equation is quite trivial ($x[n]=-1^n$).  Can we have e general equation for the other cases i.e. when there are multiple $-1$'s followed by $+1$'s ? Ubuntu desktop suddenly points to home folder