I've recently started reading the book Designing Data-Intensive Applications. This book has revealed that I have been using indexes incorrectly, in that I didn't realize that even if I have an index on two columns on a single table, my query can only use one of those indexes! For the past 3 years, I've been using indexes as if they are composable, when they are very much not so! (This is a limitation of the B-Tree index - I won't go into the details). One solution to this problem is to use a concatenated index, in which we take all the columns we want to search / filter by, combine the data into one piece of data, and then search on that combined piece of data. This likely works great in practice, but again, the indexes are not composable. If I have a concatenated index (on my persons table) on the first name and last name column, and now I want to search / filter on age as well, I have to create a new index. Are there any general solutions to this composability problem with indexes?
Which databases use real multidimensional indices? Is oracle ever using several indices to get data from tables, or will it always take the one that seems to have the highest selectivity? How about other dbms?
There is a possibility for our universe to be the surface volume of a (higher-dimensional) hyperspace. So if this possibility is true,then is there also a possibility that the other galaxies we see are our own galaxy after light has completed revolution around the three dimensional boundary (surface volume).  Know that we see different shapes for different galaxies.This can be perhaps due to other reasons like-   distance light travelled, the angle through which we look to see the other galaxy (possibly Milky Way) after     it made any number of revolutions, the part of the other galaxy (possibly Milky Way) which we see,and some other properties of light and space.   Only our Milky Way galaxy is just one possibility  Another possibilities are the number of galaxies are far less than we see because we see the same galaxy several times due to curved space.  Any possibilities for the above statements to be true? How do I call a Windows shell command using VB6? Question:  Let ${a_n}$ be a sequence in $\mathbb{R}$ with $\lim_{n\to\infty}a_n=a$  Prove that  $\lim_{n\to\infty}a_n^2=a^2$  Attempt: Without using any properties of limits (this is a question in a section of the book that came before any list of any limit theorems)  By definition:  $|a_n^2-a^2|&lt;\epsilon$   $|a_n^2-a^2|$ $\leq$ $|(a_n+a)(a_n-a)|$ $\leq$ $|(a_n+a)| |(a_n-a)|$ $\leq$   $(|a_n | + |a|)(|a_n -a|)$ $&lt;$ $\epsilon$  And since I know that $\lim_{n\to\infty}a_n=a$, I'll subsitute that in for $|a_n|^2$ as $a^2$   $\therefore$ $a^2-|a|^2 &lt; \epsilon$ which is true since $a^2-|a|^2$ in my case.  I wonder if this is a possible solution? How to manually parse a floating point number from a string Can I board if my flight arrives 1.5 hours before my UK visa becomes valid? My visa to UK is valid starting from June 25 but my flight to LGW is June 24 10:30 PM (edit: this is landing time not departure based on VCE-LGW flight times presented by easyjet.com). I still have 1.5 hours until my visa becomes valid. I prefer not to change my flight because I don't want to pay extra charges.   Will the EasyJet airlines going to allow me on board? I will be coming from Venice. How long does it take from landing to Immigration in London Gatwick? Can I stay longer in the airport before I pass through Immigration? Can I still see my luggage in the conveyor belt after 1.5 hours? How much time before they take away my luggage from the belt? How will Cryptography be changed by Quantum Computing? I realise this isn't a 'yes or no' question, and I apologise for asking something that could be seen as a discussion thread, but I had to ask.  I'm currently doing an EPQ in CS (specifically how QC will change Cryptography). I'm trying to gather up topics to cover, and so far I've scribbled down -  Effects -   National security, classical cryptographic methods (RSA, DSA, AES-256). Including Shor's algorithm. Communications Online services, e.g. BitCoin.   How would we counter this (not sure how better to phrase this) -   Lattice-based Multivariate Hash-based signatures Code-based cryptography   Is there anything else you'd recommend me including (as well as the above), or anything in the above you don't think I should include?   Honestly, the list above is a rough, first draft the main bulk of it... so don't it as gospel.  Best regards,  Cameron. Dropping Internet connection on Realtek Semiconductor Co., Ltd. RTL8821AE I have continuous problems of network dropping connections on wifi with ubuntu 16.04, I have an ASUS R510J, this is my network card:  ##### lspci #############################  03:00.0 Network controller [0280]: Realtek Semiconductor Co., Ltd. RTL8821AE 802.11ac PCIe Wireless Network Adapter [10ec:8821]         Subsystem: AzureWave RTL8821AE 802.11ac PCIe Wireless Network Adapter [1a3b:2161]         Kernel driver in use: rtl8821ae  04:00.1 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller [10ec:8168] (rev 12)         Subsystem: ASUSTeK Computer Inc. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller [1043:200f]         Kernel driver in use: r8169   sudo service network-manager restart doesnt's help, as I never regain internet connection after it drops, and I have to restart the PC. On Ethernet cable I have no problem.  What do you think is the issue? drivers?  thank you How to connect to Oracle as "SYS" from SQL*Plus in Java The complex general linear group is a subgroup of the group of real matrices of twice the dimension and with positive determinant.  Let us decompose complex matrices $M$ as $M=A+iB$, where $A,B$ are real matrices. Now consider the correspondence $$f(A+iB)=\begin{pmatrix} A &amp; -B \\ B &amp; A\end{pmatrix}.$$  If $\det f(M)=|\det M|^2$ for square matrices, then we would have $GL(n,\mathbb C)\subseteq GL_+(2n,\mathbb R)$ with the identification $M\to f(M)$, which is an injective homomorphism. In other words, the complex general linear group would be a subgroup of the group of real matrices of twice the dimension and with positive determinant.  How is $\det f(M)=|\det M|^2$? I'm trouble with exercise 1.8 of Carroll's Space-Time and Geometry:     If $\partial_\nu T^{\mu \nu} = Q^\mu$, what physically does the spatial vector $Q^i$ represent? Use the dust energy momentum tensor to make your case.    First of all, since the momentum energy tensor of dust $T^{\mu \nu}=\rho U^\mu U^\nu$ is symmetric, one has the energy-momentum conservation law $$ \partial_\nu T^{\mu \nu} = \partial_\nu T^{\nu \mu}  =0.$$ A vector $Q$ is constant in any frame related by a Lorentz transformation, while this isn't the case for the components $Q^\mu$. But here $Q=0$ and hence $Q^\mu=0$ also for every component $\mu =0,\cdots,4$?   Even if a straightforwardly apply the equation: $$ Q^i = \partial_\nu T^{i \nu} = (\partial_\nu \rho) U^i U^\nu + \rho(\partial_\nu U^i)  U^\nu + \rho U^i(\partial_\nu  U^\nu ),$$ doesn't seems to yield something...   Is my first reasoning right ? Otherwise: can someone give me a hint ?  Thanks How do I prove that $ \displaystyle\lim_{n \to{+}\infty}{\sqrt[n]{n!}}$ is infinite? How to create a squiggle arrow with some text on it in TikZ? I need to define a new command to create a squiggle arrow with some text on it. Something similar to what \xrightarrow command produces but with wiggly arrows as in \rightsquigarrow. The length of the arrow should automatically be adjusted to fit the text above it and I do not know how to handle this part in TikZ. Any ideas would be much appreciated. Looking for a (short?) story about a cryo ship looking for habitable planets I read this story/novella at least 15 years ago, probably 20 or more. It is about a ship, travelling through the universe, looking for habitable planets. Why, I can't remember. People are held in Cryo Stasis and I think they are wearing nothing in the chamber but some kind of mesh. Every time the ship gets close to a planet it wakes one or two of the people (one woman, one man?) to check it out but they never find anything good. I think in the end they wake up and discover, that they have arrived back at Earth. Other details are very vague but I seem to recall there was lots of stuff about a (blue?) light, dreaming/dreams and something about the cold, or cold blue lights or similar. Does that ring a bell for anyone? why does fdisk add one additional sector when creating partitions? I'm playing around with fdisk and I tried to create a partition on an empty USB drive. Here's how that went: Command (m for help): n Partition number (1-128, default 1): 1 First sector (34-61187102, default 2048): 2048 Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-61187102, default 61187102): +30000000           Created a new partition 1 of type 'Linux filesystem' and of size 14,3 GiB.  so just one partition with 30 million sectors. However, when I check how many sectors it contains I get:  I'm checking with a different program and I see again that additional sector:  Sorry for the pettiness :) I'm learning. Using sets of arguments from a list Why is Tony Stark in the Iron Man suit unaffected by G-forces, inertia and blunt force trauma? Tony's suit is an engineering marvel, and they explain many facets of the technology to help give legitimacy to the special powers, abilities and weapons it has.  I never read the Iron Man comics, but in the movie they really don't have a technological explanation for how he miraculously seems to be unaffected by crushing G-forces of sudden acceleration, the inertia of sudden stops and hair-pin turns, and especially the blunt force trauma from getting smashed in the face with Thor's hammer.  Certainly it appears the suit takes punishment, but Tony always remains unscatched, never a collapsed lung, broken bone or even a bloody nose.  In my infantile understanding of theoretical physics, the same technology that would be used to counter G-forces and sudden blunt force trauma to the body inside would be the very same technology used for  which in my honest opinion would be a much more stunning technological achievement than his heralded Arc Reactor.  Was their ever an explanation proferred for this and if so was it indeed attributed to Anti-Gravity? Every time Windows 10 runs updates (which I see no way to control), it updates my graphics driver.  I like the old one because it is faster.  So, I have to reinstall it.  This is getting to be a pain.  Is there any way to stop Windows 10 from updating it again? Fundamental question about dimensional analysis As a student, I make mistakes all the time, and stealing my idea will not face heavy retaliation. I don't mind if someone comes up with concurrent works, but I do mind if my key idea is leaked. Many graduate students in my field therefore only use pre-print services if their papers are either publicly presented or conditionally accepted. I am looking for a website that can do most of what ArXiv does, with an additional private option or limited visibility, for example an option to make the paper unindexed by search engines. This way, my paper cannot be seen for at least a few months. By doing so, I can at least claim that I am one of the first few researchers who got the idea right. And most importantly, I can privately correct my mistakes, if any. Publishing mistakes is embarrassing and sometimes harms one's career. Many mistakes will be revealed during the peer-review processes and I have a chance to correct them before publicizing. Please kindly think twice before giving ethically and &quot;politically correct&quot; comments like no one will scoop me, everyone will cite me, and the scoopers will be punished. At least in my field, scooping and idea-stealing is commonly observable, and big-names don't care about publicizing pre-prints because they have resources to punish the scoopers. I do understand that keeping my work private might harm the community as the information becomes less transparent. However, it could also be possible that my un-reviewed flawed works misguide the community, and a paper reviewed will benefit the society more. I personally wasted six months because of someone else's mistake in a published work.  I heard that in molecular biology there is a widely-used service to upload your research data, like protein structure and sequencing, to get a timestamp. The uploaded materials can be set to be private for a period, and fully disclosed later. A lot of people do use this function. This is not exactly what I am looking for but shares some similar ideas. If I remember correctly, there is an interesting paper studying the scooping patterns with the aid of this database.  Solutions found!  and some other university-run pre-print services do have an embargo function. Moreover,  is a widely used repository, that, unlike arxiv, only displays the newest version to the general public, although still keeps a tag on the first submitted date! Ball swung in a horizontal circle, can the string ever by exactly horizontal to the ground? You swing a ball on the end of a lightweight string in a horizontal circle at constant speed.  Can the string ever by truly horizontal?  If not, would it slope above the horizontal or below the horizontal?  Why?