Using Postgres 9.4. I have a table cartests with 5.5M rows. Each row is a car test:  \d log.cartests;              Table "log.cartests"    Column    |           Type           | Modifiers  -------------+--------------------------+-----------  carid       | integer                  | not null  timestamp   | timestamp with time zone | not null  exhaust     | varchar                  |   brakes      | varchar   Indexes:     "cartests_pkey" PRIMARY KEY, btree (id, "position", "timestamp")     "cartests_carid_idx" btree (carid)     "cartests_timestamp_idx" btree ("timestamp")   I need to retrieve the latest log for each car.  SELECT DISTINCT ON (carid) carid, timestamp, exhaust, brakes FROM log.cartests ORDER BY carid, timestamp DESC;   This one takes ~ 6 seconds. Running EXPLAIN ANALYZE gives:   Unique  (cost=933787.06..961490.34 rows=472 width=16)           (actual time=4951.347..6072.534 rows=476 loops=1)   -&gt;  Sort  (cost=933787.06..947638.70 rows=5540656 width=16)              (actual time=4951.345..5782.466 rows=5540656 loops=1)         Sort Key: carid, "timestamp"         Sort Method: external merge  Disk: 162504kB         -&gt;  Seq Scan on cartests               (cost=0.00..123810.56 rows=5540656 width=16)             (actual time=0.009..620.988 rows=5540656 loops=1) Planning time: 0.107 ms Execution time: 6095.598 ms    Why must it sort if I have an index on both carid and timestamp? Is there a better method to use here?
I have a query pattern that must be very common, but I don't know how to write an efficient query for it.  I want to look up the rows of a table that correspond to "the most recent date not after" the rows of another table.  I have a table, inventory say, which represents the inventory I hold on a certain day.  date       | good | quantity ------------------------------ 2013-08-09 | egg  | 5 2013-08-09 | pear | 7 2013-08-02 | egg  | 1 2013-08-02 | pear | 2   and a table, "price" say, which holds the price of a good on a given day  date       | good | price -------------------------- 2013-08-07 | egg  | 120 2013-08-06 | pear | 200 2013-08-01 | egg  | 110 2013-07-30 | pear | 220   How can I efficiently get the "most recent" price for each row of the inventory table, i.e.  date       | pricing date | good | quantity | price ---------------------------------------------------- 2013-08-09 | 2013-08-07   | egg  | 5        | 120 2013-08-09 | 2013-08-06   | pear | 7        | 200 2013-08-02 | 2013-08-01   | egg  | 1        | 110 2013-08-02 | 2013-07-30   | pear | 2        | 220   I know one way of doing this:  select inventory.date, max(price.date) as pricing_date, good from inventory, price where inventory.date &gt;= price.date and inventory.good = price.good group by inventory.date, good   and then join this query again to inventory. For large tables even doing the first query (without joining again to inventory) is very slow.  However, the same problem is quickly solved if I simply use my programming language to issue one max(price.date) ... where price.date &lt;= date_of_interest ... order by price.date desc limit 1 query for each date_of_interest from the inventory table, so I know there is no computational impediment.  I would, however, prefer to solve the whole problem with a single SQL query, because it would allow me to do further SQL processing on the result of the query.  Is there a standard way to do this efficiently? It feels like it must come up often and that there should be a way to write a fast query for it.  I'm using Postgres, but an SQL-generic answer would be appreciated.
How to prove that exponential grows faster than polynomial? How can I drive 12v LEDs from my Arduino Nano? Background I am building a scale RC helicopter and ordered a bunch of 3mm LEDs to give it some nice navigation and strobe lights which I plan to drive with an Arduino Nano. Problem is that I goofed and didn't notice that the LEDs are 12v until they arrived.  I know that the Arduino Nano can accept 12V on pin 30, but I think the voltage output on any of the &quot;D&quot; or &quot;A&quot; pins is always 5v (correct??). Question If the voltage output of the Arduino pins is 5v, then how can I drive the 12v LEDs using the Arduino, or is it impossible? Which is correct: "drive safe" or "drive safely"? When someone is going to drive their car somewhere, I always used to say "drive safely" to them. Recently I was told I should say "drive safe."    (From: )   Which one is correct?  Similarly, is "do good" correct? How to move a database from SQL Server 2012 to SQL Server 2005 What are my options if I need to move a database from SQL Server 2012(32bit) to SQL Server 2005(64bit)?  I know I cannot:   restore a backup of the database on SQL Server 2005 detach &amp; attach   I know I can:   use import data wizard, and I tried it on one database, but it only moves data, and even that was troublesome as I needed to do a lot of work creating temp tables to maintain identity columns, recreating all FKs, indexes etc.    Is there an easier option? QGIS color polygon according to points I have two layers. Layer 1 contains points (x,y) and layer 2 contains one polygon. All points in layer 1 are positioned within the polygon of layer 2. The points in layer 1 contain an attribute with information on the topography/elevation at the specific point. Can I color the polygon in layer 2 based on attributes from the points in layer 1 in order to create an elevation map? How to distinguish between gold, silver, and bronze tag badges on SEDE? Using Sequence to delete a part of an expression If Voldemort's body dies, what process would he use to restore himself from a horcrux? His plans seem sloppy for the pains he had taken. For example, why create horcruxes from pieces of jewellery when they dont have the agency to seek out a new body by themselves?  He needed Quirrel, a person with unlikely ambitions to seek him out to gain an useful body. When that body was gone, he needed Barty Crouch, someone with questionable agency (he could be imprisoned) to transport Harry and draw his blood.  Nagini and the diary seems the only horcruxes with agency, the others are mostly inert.  What plans were ever taken to restore Voldemort from his horcruxes? WinForms: How to programmatically fire an event handler? I want to programmatically invoke an event handler for a control. For example:  DateTimePicker dtpLastConsummated;   I want to trigger the TextChanged event handler for the dtpLastConsummated, how can i do it?  In other languages I would call something akin to:  dtpLastConsummated.TextChanged(this, new EventArgs());   but in .NET you can have multiple event handlers:  dtpLastConsummated.Click +=new EventHandler(dtpLastConsummated_TextChanged); dtpLastConsummated.Click +=new EventHandler(dtpLastConsummated_AnotherHandler); dtpLastConsummated.Click +=new EventHandler(dtpLastConsummated_MoreHandlers); ... dtpLastConsummated.Click +=new EventHandler(dtpLastConsummated_Nminus1);   so you need a way to trigger all the attached event handlers.    Answer  The following code will fire the event:  Toolkit.FireEvent(dtpLastConsummated, "TextChanged", new EventArgs());   And here's the code of the static toolkit function:  /// &lt;summary&gt; /// Programatically fire an event handler of an object /// &lt;/summary&gt; /// &lt;param name="targetObject"&gt;&lt;/param&gt; /// &lt;param name="eventName"&gt;&lt;/param&gt; /// &lt;param name="e"&gt;&lt;/param&gt; public static void FireEvent(Object targetObject, string eventName, EventArgs e) {    /*     * By convention event handlers are internally called by a protected     * method called OnEventName     * e.g.     *     public event TextChanged     * is triggered by     *     protected void OnTextChanged     *      * If the object didn't create an OnXxxx protected method,     * then you're screwed. But your alternative was over override     * the method and call it - so you'd be screwed the other way too.     */     //Event thrower method name //e.g. OnTextChanged    String methodName = "On" + eventName;     MethodInfo mi = targetObject.GetType().GetMethod(          methodName,           BindingFlags.Instance | BindingFlags.NonPublic);     if (mi == null)       throw new ArgumentException("Cannot find event thrower named "+methodName);     mi.Invoke(targetObject, new object[] { e }); }     Note: I'm not creating a subclass of every control in the .NET framework, and every 3rd party control, and convincing an enterprise worth of developers to retrofit every form to use my custom controls. Persistent Dummy Output The soundcard I want to use is detected and works (it only recently switched to dummy output and didn't allow me to go back). HDMI is no longer listed under Audio and Video - KDE Control Module; everything under audio module is set to dummy output and its the only 'soundcard' on the list now. I can view my previous sound cards in alsamixer, but changing the settings from there does nothing. How can I get my real soundcards to work again? I have already tried the following answers without success:  EDIT: name@name-desktop:~$ lspci -vvv | grep -A8 Audio 00:01.1 Audio device: Advanced Micro Devices, Inc. [AMD/ATI] Kaveri HDMI/DP Audio Controller         Subsystem: ASRock Incorporation Kaveri HDMI/DP Audio Controller         Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+         Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast &gt;TAbort- &lt;TAbort- &lt;MAbort- &gt;SERR- &lt;PERR- INTx-         Latency: 0, Cache Line Size: 64 bytes         Interrupt: pin B routed to IRQ 43         Region 0: Memory at ff764000 (64-bit, non-prefetchable) [size=16K]         Capabilities: &lt;access denied&gt;         Kernel driver in use: snd_hda_intel         Kernel modules: snd_hda_intel -- 00:14.2 Audio device: Advanced Micro Devices, Inc. [AMD] FCH Azalia Controller (rev 01)         Subsystem: ASRock Incorporation FCH Azalia Controller         Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-         Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=slow &gt;TAbort- &lt;TAbort- &lt;MAbort- &gt;SERR- &lt;PERR- INTx-         Latency: 32, Cache Line Size: 64 bytes         Interrupt: pin A routed to IRQ 16         Region 0: Memory at ff760000 (64-bit, non-prefetchable) [size=16K]         Capabilities: &lt;access denied&gt;         Kernel driver in use: snd_hda_intel  Could this image possibly help?:  There is another soundcard but I'm unsure of what its for because it never outputs sound, but it does have a lot of settings: that sound card is labelled as &quot;HD-Audio Generic&quot;.   So I went into Software &amp; Updates under the Additional drivers tab and I see this:  It was on Do not use the device so I changed it and applied the settings but unfortunately the soundcard isn't back even after reboot. Finder sidebar flashing text and high CPU load since El Capitan After update to OS X 10.11 I started having these bugs:   Finder sidebar text flashing  Very high CPU load:      I have Macbook Pro Retina 13" Mid 2014  I reset NVRAM.  I deleted com.apple.finder.plist, it didn't help.  Also I did this: , no longer flashes. Find $P(n+1)$ for a polynomial P Fix a nonnegative integer $n$. Let $P(x)$ be a polynomial of degree $n$ (over the real numbers) such that for all $k\in\left\{0,1,\ldots,n\right\}$, we have $P(k)=\dfrac{k}{k+1}$. Find $P(n+1)$.   My attempt:  I consider a new polynomial $Q(x)=(x+1)P(x)-x$. This $Q$ is a polynomial of degree $\leq n+1$ satisfying: $Q(k)=0\quad\forall k\in\{0,1,2,\ldots,n\}$. Therefore $Q(x)=\lambda x(x-1)\cdots(x-n)$ for a certain real $\lambda$. Hence, $Q(-1)=\lambda \cdot (-1)^{n+1}(n+1)!$, but also $Q(-1)=(-1+1)P(-1)+1=1$. Comparing these, we get $\lambda \cdot (-1)^{n+1} (n+1)! = 1$, thus $\lambda=\dfrac{(-1)^{n+1}}{(n+1)!}$. Since $Q(n+1)=\lambda(n+1)!$ then $Q(n+1)=(-1)^{n+1} $, and from $P(x) = \dfrac{Q(x)+x}{x+1}$ we deduce: $$P(n+1)=\dfrac{(-1)^{n+1}+n+1}{n+2}.$$ I doubted that the answer were $P(n+1)=\frac{n+1}{n+2}$ but the term $(-1)^{n+1}$ wasn’t expected. Is there any mistake here? Does someone have an alternative proof? I took several statistics courses in college but I found that my education was very theory driven.  I was wondering if any of you had a text in Applied Statistics (at the graduate level) that you recommend or have had good experience with. Query to collect the recent TimeSampled row using inner joins in MS SQL Watching The Lord of the Rings movies I was really captivated by the   elephant riders and the men in brazen scale armor who aided the orcs   against the forces of Gondor and Rohan. From what I know they are from   the cultures that worship Sauron. I'd like to know what Tolkien wrote   about them, their religion, their way of life, and so on.   From How can I identify hardware components in a laptop? I just reinstalled my Windows and now I don't have all the hardware components recognized. I have an  laptop and  website seems to have all the necessary drivers for my model. The problem is that I don't know exactly which components I have. For example, in my laptop video card case the site has available drivers for ATI and Intel chips.   How do I know what are the right drivers for my machine? Convert a JSON string to object in Java ME? What term describes a non-offensive substitute for a swear word?   For example, Battlestar Galactica used frack instead of fuck. Another example is the use of snap instead of shit.  I think I may have heard a single-word term used to describe such alternate non-offensive words before, but I don’t know what it is. Story about an intelligent, walled city; people are reproduced by machines, protagonist rebels and tries to leave Search engines and browser accept-language I'm building a web portal where language content will generally depend on the "accept-language" sent by the browser. The same content-URI will thus serve different content to different users depending on their browser setting.  I'm very curious to know how this will affect search indexing. Does Google index using all languages, and is it handled well?