You ask:

&gt; Is there a command that can change this maximum length? Is this length
&gt; the same for other names (columns, tables etc)?

The [manual answers here][1]:
 
&gt; The system uses no more than `NAMEDATALEN-1` bytes of an identifier;
&gt; longer names can be written in commands, but they will be truncated.
&gt; By default, `NAMEDATALEN` is 64 so the maximum identifier length is **63 bytes**. If this limit is problematic, it can be raised by changing the
&gt; `NAMEDATALEN` constant in `src/include/pg_config_manual.h`.

Bold emphasis mine.

The only way to change it is to hack the source code and recompile PostgreSQL.  
Domain names are identifiers like any other. When I execute:

    CREATE DOMAIN d_complement_activite_etablissement_or_even_loger_than_that AS text

I get what I ordered (tested on PostgreSQL 8.4 - 11):

    d_complement_activite_etablissement_or_even_loger_than_that

There must be some other piece of software between you and your database cropping the name.

Note that characters in UTF encoding occupy **1 to 4 bytes**. (But ASCII characters (like used in the example) are encoded with a single byte.)

  [1]: https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS

I have two classes that perform date date range data fetching for particular days.

    public class IterationLookup&lt;TItem&gt;
    {
        private IList&lt;Item&gt; items = null;
    
        public IterationLookup(IEnumerable&lt;TItem&gt; items, Func&lt;TItem, TKey&gt; keySelector)
        {
            this.items = items.OrderByDescending(keySelector).ToList();
        }
    
        public TItem GetItem(DateTime day)
        {
            foreach(TItem i in this.items)
            {
               if (i.IsWithinRange(day))
               {
                   return i;
               }
            }
            return null;
        }
    }
    
    
    public class LinqLookup&lt;TItem&gt;
    {
        private IList&lt;Item&gt; items = null;
    
        public IterationLookup(IEnumerable&lt;TItem&gt; items, Func&lt;TItem, TKey&gt; keySelector)
        {
            this.items = items.OrderByDescending(keySelector).ToList();
        }
    
        public TItem GetItem(DateTime day)
        {
            return this.items.FirstOrDefault(i =&gt; i.IsWithinRange(day));
        }
    }

Then I do speed tests which show that Linq version is about **5 times slower**. This would make sense when I would store items locally without enumerating them using `ToList`. This would make it much slower, because with every call to `FirstOrDefault`, linq would also perform `OrderByDescending`. But that&#39;s not the case so I don&#39;t really know what&#39;s going on. **Linq should perform very similar to iteration.**

This is the code excerpt that measures my timings
--

    IList&lt;RangeItem&gt; ranges = GenerateRanges(); // returns List&lt;T&gt;

    var iterLookup = new IterationLookup&lt;RangeItems&gt;(ranges, r =&gt; r.Id);
    var linqLookup = new LinqLookup&lt;RangeItems&gt;(ranges, r =&gt; r.Id);

    Stopwatch timer = new Stopwatch();

    timer.Start();
    for(int i = 0; i &lt; 1000000; i++)
    {
        iterLookup.GetItem(GetRandomDay());
    }
    timer.Stop();
    // display elapsed time
    
    timer.Restart();
    for(int i = 0; i &lt; 1000000; i++)
    {
        linqLookup.GetItem(GetRandomDay());
    }
    timer.Stop();
    // display elapsed time

Why do I know it should perform better? Because when I write a very similar code without using these lookup classes, Linq performs very similar to `foreach` iterations...

    // continue from previous code block

    // items used by both order as they do in classes as well
    IList&lt;RangeItem&gt; items = ranges.OrderByDescending(r =&gt; r.Id).ToList();
    
    timer.Restart();
    for(int i = 0; i &lt; 1000000; i++)
    {
        DateTime day = GetRandomDay();
        foreach(RangeItem r in items)
        {
            if (r.IsWithinRange(day))
            {
                // RangeItem result = r;
                break;
            }
        }
    }    
    timer.Stop();
    // display elapsed time

    timer.Restart();
    for(int i = 0; i &lt; 1000000; i++)
    {
       DateTime day = GetRandomDay();
       items.FirstOrDefault(i =&gt; i.IsWithinRange(day));
    }
    timer.Stop();
    // display elapsed time

This is by my opinion highly similar code. `FirstOrDefault` as much as I know also iterate for only as long until it gets to a valid item or until it reaches the end. And this is somehow the same as `foreach` with `break`.

But even iteration class performs worse than my simple `foreach` iteration loop which is also a mystery because all the overhead it has is the call to a method within a class compared to direct access.

Question
--
What am I doing wrong in my LINQ class that it performs so very slow?  
What am I doing wrong in my Iteration class so it performs twice as slow as direct `foreach` loop?

Which times are being measured?
--
I do these steps:

1. Generate ranges (as seen below in results)
2. Create object instances for IterationLookup, LinqLookup (and my optimized date range class BitCountLookup which is not part of discussion here)
3. Start timer and execute 1 million lookups on random days within maximum date range (as seen in results) by using previously instantiated IterationLookup class.
4. Start timer and execute 1 million lookups on random days within maximum date range (as seen in results) by using previously instantiated LinqLookup class.
5. Start timer and execute 1 million lookups (6 times) using manual foreach+break loops and Linq calls.

As you can see **object instantiation is not measured**.

Appendix I: Results over million lookups
--
Ranges displayed in these results don&#39;t overlap, which should make both approaches even more similar in case LINQ version doesn&#39;t break loop on successful match (which it highly likely does).

&lt;pre&gt;
Generated Ranges:

ID Range        000000000111111111122222222223300000000011111111112222222222
                123456789012345678901234567890112345678901234567890123456789
09 22.01.-30.01.                     |-------|
08 14.01.-16.01.             |-|
07 16.02.-19.02.                                              |--|
06 15.01.-17.01.              |-|
05 19.02.-23.02.                                                 |---|
04 01.01.-07.01.|-----|
03 02.01.-10.01. |-------|
02 11.01.-13.01.          |-|
01 16.01.-20.01.               |---|
00 29.01.-06.02.                            |-------|

Lookup classes...

- Iteration: 1028ms
&lt;b&gt;- Linq: 4517ms   !!! THIS IS THE PROBLEM !!!&lt;/b&gt;
- BitCounter: 401ms

Manual loops...

- Iter: 786ms
- Linq: 981ms
- Iter: 787ms
- Linq: 996ms
- Iter: 787ms
- Linq: 977ms
- Iter: 783ms
- Linq: 979ms
&lt;/pre&gt;

Appendix II: GitHub:Gist code to test for yourself
--
I&#39;ve put up a Gist so you can get the full code yourself and see what&#39;s going on. Create a **Console** application and copy *Program.cs* into it an add other files that are part of this gist.

Grab it [here](https://gist.github.com/1385081).

Appendix III: Final thoughts and measurement tests
--
The most problematic thing was of course LINQ implementatino that was awfully slow. Turns out that this has all to do with delegate compiler optimization. [LukeH provided the best and most usable solution](https://stackoverflow.com/q/8225900/75642) that actually made me try different approaches to this. I&#39;ve tried various different approaches in the `GetItem` method (or `GetPointData` as it&#39;s called in Gist):

1. the usual way that most of developers would do (and is implemented in Gist as well and wasn&#39;t updated after results revealed this is not the best way of doing it):

        return this.items.FirstOrDefault(item =&gt; item.IsWithinRange(day));

2. by defining a local predicate variable:

        Func&lt;TItem, bool&gt; predicate = item =&gt; item.IsWithinRange(day);
        return this.items.FirstOrDefault(predicate);

3. local predicate builder:

        Func&lt;DateTime, Func&lt;TItem, bool&gt;&gt; builder = d =&gt; item =&gt; item.IsWithinRange(d);
        return this.items.FirstOrDefault(builder(day));

4. local predicate builder and local predicate variable:


        Func&lt;DateTime, Func&lt;TItem, bool&gt;&gt; builder = d =&gt; item =&gt; item.IsWithinRange(d);
        Func&lt;TItem, bool&gt; predicate = builder(day);
        return this.items.FirstOrDefault(predicate);

5. class level (static or instance) predicate builder:

        return this.items.FirstOrDefault(classLevelBuilder(day));

6. externally defined predicate and provided as method parameter

        public TItem GetItem(Func&lt;TItem, bool&gt; predicate)
        {
            return this.items.FirstOrDefault(predicate);
        }

    when executing this method I also took two approaches:

    1. predicate provided directly at method call within `for` loop:

            for (int i = 0; i &lt; 1000000; i++)
            {
                linqLookup.GetItem(item =&gt; item.IsWithinRange(GetRandomDay()));
            }

    2. predicate builder defined outside `for` loop:

            Func&lt;DateTime, Func&lt;Ranger, bool&gt;&gt; builder = d =&gt; r =&gt; r.IsWithinRange(d);
            for (int i = 0; i &lt; 1000000; i++)
            {
                linqLookup.GetItem(builder(GetRandomDay()));
            }

###Results - what performs best
For comparison when using iteration class, it takes it approx. **770ms** to execute 1 million lookups on randomly generated ranges.

1. #3 local predicate builder turns out to be best compiler optimized so it performs almost as fast as usual iterations. **800ms**.
2. #6.2 predicate builder defined outside `for` loop: **885ms**
3. #6.1 predicate defined within `for` loop: **1525ms**
4. all others took somewhere between **4200ms - 4360ms** and are thus considered unusable.

So whenever you use a predicate in externally frequently callable method, define a builder and execute that. This will yield best results.

The biggest surprise to me about this is that delegates (or predicates) may be this much time consuming.


foreach + break vs linq FirstOrDefault performance difference

What are the default values for the following options in Java 6 on a *non-server-class machine*?

 - -Xmx
 - -XX:MaxPermSize

Oracle&#39;s [documentation][1] states that:

&gt; On server-class machines running the server VM, the garbage collector (GC) has changed from the previous serial collector (-XX:+UseSerialGC) to a parallel collector (-XX:+UseParallelGC).

and

&gt; On server-class machines running either VM (client or server) with the parallel garbage collector (-XX:+UseParallelGC) the initial heap size and maximum heap size have changed

The page doesn&#39;t describe the defaults for non-server-class machines, only that, for example, the initial heap size is &quot;a reasonable minimum&quot;. Looking at [the &#39;man page&#39;][2] for the `java` command there is the following against `-Xms`:

&gt; The default value is chosen at runtime based on system configuration 


  [1]: http://docs.oracle.com/javase/6/docs/technotes/guides/vm/gc-ergonomics.html
  [2]: http://docs.oracle.com/javase/6/docs/technotes/tools/windows/java.html

Default values for Xmx, Xms, MaxPermSize on non-server-class machines

Hope this question wasn&#39;t asked before. Does anyone know the character limit for domain names?
For example if I write this:

    CREATE DOMAIN d_complement_activite_etablissement AS character varying

it will create a domain with the name:

    d_complement_activite_etabliss

(Yeah, I know how to count, but I want some more info on the subject).

Is there a command that can change this maximum length? Is this length the same for other names (columns, tables etc)?

Maximum characters in labels (table names, columns etc)

Hope this question wasn't asked before. Does anyone know the character limit for domain names?
For example if I write this:
<pre><code class="hljs language-sql">CREATE DOMAIN d_complement_activite_etablissement AS character varying
</code></pre>
it will create a domain with the name:
<pre><code class="hljs">d_complement_activite_etabliss
</code></pre>
(Yeah, I know how to count, but I want some more info on the subject).
Is there a command that can change this maximum length? Is this length the same for other names (columns, tables etc)?

Does PostgreSQL support computed / calculated columns, like MS SQL Server?  I can&#39;t find anything in the docs, but as this feature is included in many other DBMSs I thought I might be missing something.

Eg: http://msdn.microsoft.com/en-us/library/ms191250.aspx

Computed / calculated / virtual / derived columns in PostgreSQL

I want to verify the syntax of files containing sql queries before they can be committed in my CVS project. 

In order to do that, I have a commitinfo script, but I have trouble finding out if the sql commands are valid. &lt;code&gt;psql&lt;/code&gt; does not seem to have a dryrun mode, and constructing my own postgresql-dialact tester from the grammar (that is in the source) seems like a long stretch. 

The scripts may contain multiple queries, so an &lt;code&gt;EXPLAIN&lt;/code&gt; cannot be wrapped around them. 

Any hints?

PostgreSQL syntax check without running the query

Does anyone know if it&#39;s even possible (and how, if yes) to query a database server setting in PostgreSQL (9.1)?

I need to check the `max_connections` (maximum number of open db connections) setting.

Query a parameter (postgresql.conf setting) like &quot;max_connections&quot;

I have a table with this layout:

    CREATE TABLE Favorites
    (
      FavoriteId uuid NOT NULL PRIMARY KEY,
      UserId uuid NOT NULL,
      RecipeId uuid NOT NULL,
      MenuId uuid
    )

I want to create a unique constraint similar to this:

    ALTER TABLE Favorites
    ADD CONSTRAINT Favorites_UniqueFavorite UNIQUE(UserId, MenuId, RecipeId);

However, this will allow multiple rows with the same `(UserId, RecipeId)`, if `MenuId IS NULL`.  I want to allow `NULL` in `MenuId` to store a favorite that has no associated menu, but I only want at most one of these rows per user/recipe pair.

The ideas I have so far are:

 1. Use some hard-coded UUID (such as all zeros) instead of null.  
However, `MenuId` has a FK constraint on each user&#39;s menus, so I&#39;d then have to create a special &quot;null&quot; menu for every user which is a hassle.

 2. Check for existence of a null entry using a trigger instead.  
I think this is a hassle and I like avoiding triggers wherever possible. Plus, I don&#39;t trust them to guarantee my data is never in a bad state.

 3. Just forget about it and check for the previous existence of a null entry in the middle-ware or in a insert function, and don&#39;t have this constraint.

I&#39;m using Postgres 9.0.

Is there any method I&#39;m overlooking?

Create unique constraint with null columns

I need to delete about 2 million rows from my PG database. I have a list of IDs that I need to delete. However, any way I try to do this is taking days.  

I tried putting them in a table and doing it in batches of 100. 4 days later, this is still running with only 297268 rows deleted. (I had to select 100 id&#39;s from an ID table, delete where IN that list, delete from ids table the 100 I selected).

I tried:

    DELETE FROM tbl WHERE id IN (select * from ids)

That&#39;s taking forever, too. Hard to gauge how long, since I can&#39;t see it&#39;s progress till done, but the query was still running after 2 days.

Just kind of looking for the most effective way to delete from a table when I know the specific ID&#39;s to delete, and there are millions of IDs.

Best way to delete millions of rows by ID

I have a very simple question - when should we apply the new keyword when creating objects in Scala? Is it when we try to instantiate Java objects only?

&quot;new&quot; keyword in Scala

I was making my way through the Scala playframework tutorial and I came across this snippet of code which had me puzzled: 

    def newTask = Action { implicit request =&gt;
    taskForm.bindFromRequest.fold(
            errors =&gt; BadRequest(views.html.index(Task.all(), errors)),
            label =&gt; {
              Task.create(label)
              Redirect(routes.Application.tasks())
            } 
      )
    }

So I decided to investigate and came across [this post][1].

I still don&#39;t get it.

What is the difference between this: 

    implicit def double2Int(d : Double) : Int = d.toInt

and

    def double2IntNonImplicit(d : Double) : Int = d.toInt

other than the obvious fact they have different method names.

When should I use `implicit` and why?

  [1]: http://pietrowski.info/2009/07/scala-implicit-conversion/

Understanding implicit in Scala

In python there is a `pass` keyword for defining an empty function, condition, loop, ...
Is there something similar for Ruby?

Python Example:

    def some_function():
        # do nothing
        pass

Equivalent of “pass” in Ruby

I have read the official definition of &quot;raise&quot;, but I still don&#39;t quite understand what it does. 

In simplest terms, what is &quot;raise&quot;?

Example usage would help.

How to use &quot;raise&quot; keyword in Python

Many of the books that I am reading use keyword `const` when the value of a variable should not be modified. Apart from specifying to readers of the code that you may cause errors if you modify this variable (you can use comments to do this), why would you need that keyword to be a part of any programming language? It seems to me that if you don&#39;t want a variable modified, simply don&#39;t.

Could someone clarify this for me? 

Why would you use the keyword const if you already know variable should be constant?

I fetch data from Google&#39;s AdWords website which has multiple elements with the same `id`.

Could you please explain why the following 3 queries doesn&#39;t result with the same answer (2)?

[Live Demo][1]

HTML:

    &lt;div&gt;
        &lt;span id=&quot;a&quot;&gt;1&lt;/span&gt;
        &lt;span id=&quot;a&quot;&gt;2&lt;/span&gt;
        &lt;span&gt;3&lt;/span&gt;
    &lt;/div&gt;

JS:

    $(function() {
        var w = $(&quot;div&quot;);
        console.log($(&quot;#a&quot;).length);            // 1 - Why?
        console.log($(&quot;body #a&quot;).length);       // 2
        console.log($(&quot;#a&quot;, w).length);         // 2
    });


  [1]: http://jsfiddle.net/P2j3f/1/

How does jQuery work when there are multiple elements with the same ID value?

Recently started digging in to JSON, and I&#39;m currently trying to use a number as &quot;identifier&quot;, which doesn&#39;t work out too well. `foo:&quot;bar&quot;` works fine, while `0:&quot;bar&quot;` doesn&#39;t. 

    var Game = {
    	status: [
                    {
                        0:&quot;val&quot;,
                        1:&quot;val&quot;,
                        2:&quot;val&quot;
                    },
                    {
                        0:&quot;val&quot;,
                        1:&quot;val&quot;,
                        2:&quot;val&quot;
                    }
               ]
    }
    
    alert(Game.status[0].0);

Is there any way to do it the following way? Something like `Game.status[0].0` Would make my life way easier. Of course there&#39;s other ways around it, but this way is preferred.

Using number as &quot;index&quot; (JSON)

I am trying to create a table that was dropped previously.  

But when I do the `CREATE TABLE A ..`. I am getting below error: 

&gt; Relation &#39;A&#39; already exists.

I verified doing `SELECT * FROM A`, but then I got another error: 

&gt; Relation &#39;A&#39; does not exists.

I already tried to find it in `\dS+` listing all relations, and it is not there.  
To complicate this, I have tested this by creating this table in another database and I got the same error. I am thinking that could be an error when this table was dropped. Any ideas?

Here is the code: I&#39;m using a generated code from Power SQL. I have the same error without using the sequence. It just works when I change the name and in this case I can not do that.

    CREATE SEQUENCE csd_relationship_csd_relationship_id_seq;
    CREATE TABLE csd_relationship (
        csd_relationship_id INTEGER NOT NULL DEFAULT nextval(&#39;csd_relationship_csd_relationship_id_seq&#39;::regclass),  
        type_id INTEGER NOT NULL,
        object_id INTEGER NOT NULL,
        CONSTRAINT csd_relationship PRIMARY KEY (csd_relationship_id)
    );

PostgreSQL Error: Relation already exists

I want to pass a table name as a parameter in a Postgres function. I tried this code:

    CREATE OR REPLACE FUNCTION some_f(param character varying) RETURNS integer 
    AS $$
        BEGIN
    	IF EXISTS (select * from quote_ident($1) where quote_ident($1).id=1) THEN
    	 return 1;
    	END IF;
        return 0;
        END;
    $$ LANGUAGE plpgsql;
    
    select some_f(&#39;table_name&#39;);

And I got this:

    ERROR:  syntax error at or near &quot;.&quot;
    LINE 4: ...elect * from quote_ident($1) where quote_ident($1).id=1)...
                                                                 ^
    
    ********** Error **********
    
    ERROR: syntax error at or near &quot;.&quot;

And here is the error I got when changed to this `select * from quote_ident($1) tab where tab.id=1`:

    ERROR:  column tab.id does not exist
    LINE 1: ...T EXISTS (select * from quote_ident($1) tab where tab.id...

Probably, `quote_ident($1)` works, because without the `where quote_ident($1).id=1` part I get `1`, which means something is selected. Why may the first `quote_ident($1)` work and the second one not at the same time? And how could this be solved?

Table name as a PostgreSQL function parameter

&lt;pre&gt;
Caused by: org.springframework.orm.hibernate3.HibernateSystemException: ids for this class must be manually assigned before calling save(): com.rfid.model.Role; nested exception is org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): com.rfid.model.Role
	at org.springframework.orm.hibernate3.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:676)
	at org.springframework.orm.hibernate3.HibernateAccessor.convertHibernateAccessException(HibernateAccessor.java:412)
	at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:424)
	at org.springframework.orm.hibernate3.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:374)
	at org.springframework.orm.hibernate3.HibernateTemplate.saveOrUpdate(HibernateTemplate.java:748)
	at com.wfos.engine.wrapper.domain.impl.WrapperImpl.save(WrapperImpl.java:159)
	... 47 more
    Caused by: org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): com.rfid.model.Role
	at org.hibernate.id.Assigned.generate(Assigned.java:53)
	at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:117)
	at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93)
	at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:685)
	at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:677)
	at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:673)
	at org.springframework.orm.hibernate3.HibernateTemplate$16.doInHibernate(HibernateTemplate.java:751)
	at org.springframework.orm.hibernate3.HibernateTemplate.doExecute(HibernateTemplate.java:419)
	... 50 more
    WARN [21:14:21] (CommonsLoggingOutput.java:59): - --Erroring: batchId[1] message[java.lang.reflect.UndeclaredThrowableException]
&lt;/pre&gt;


My class is like this:

    @Entity
    @javax.persistence.Table(name=&quot;Role&quot;)
    @Table(appliesTo = &quot;Role&quot;)
    public class Role {
	
	@Id  
	@Column(name=&quot;U_id&quot;)
	public String U_id;
	  
	public String U_pwd;
	 
	public String U_account;
	  
	public String U_mode;
	 
	public String U_status;


	public String getU_pwd() {
		return U_pwd;
	}

	public void setU_pwd(String u_pwd) {
		U_pwd = u_pwd;
	}

	public String getU_account() {
		return U_account;
	}

	public void setU_account(String u_account) {
		U_account = u_account;
	}

	public String getU_id() {
		return U_id;
	}

	public void setU_id(String u_id) {
		U_id = u_id;
	}

	public String getU_mode() {
		return U_mode;
	}

	public void setU_mode(String u_mode) {
		U_mode = u_mode;
	}

	public String getU_status() {
		return U_status;
	}

	public void setU_status(String u_status) {
		U_status = u_status;
	}
	
    }

Content Type	Original Author	Original Content on Stackoverflow
Question	Florin Vistig	View Question on Stackoverflow
Solution 1 - Postgresql	Erwin Brandstetter	View Answer on Stackoverflow

Maximum characters in labels (table names, columns etc)

Postgresql Problem Overview

Postgresql Solutions

Solution 1 - Postgresql

Default values for Xmx, Xms, MaxPermSize on non-server-class machines

foreach + break vs linq FirstOrDefault performance difference

Attributions