Loops Versus Apply: Speed Comparison

Kevin Feasel

2018-02-19

R

Mike Spencer compares lapply (single core and its multi-core version) versus a for loop in R:

But how fast were they? Can we get faster? Thankfully R provides `system.time()` for timing code execution. In order to get faster, it makes sense to use all the processing power our machines have. The ‘parallel’ library has some great tools to help us run our jobs in parallel and take advantage of multicore processing. My favourite is `mclapply()`, because it is very very easy to take an `lapply` and make it multicore. Note that mclapply doesn’t work on Windows. The following script runs the `read_clean_write()` function in a for loop (boo, hiss), lapply and mclapply. I’ve run these as list elements to make life easier later on.

It’s interesting reading, particularly because I had expected lapply to do a little bit better.  Also interesting is the relative overhead cost of mclapply in this scenario:  going from 1 core to 4 cut the time to approximately 1/3, not 1/4.

Quoted Concatenation In R

Kevin Feasel

2018-02-19

R

John Mount has a quick tip for R users:

Here is an R tip. Need to quote a lot of names at once? Use qc().

This function is part of wrapr.

Pushing SSH User Keys With Powershell

Anthony Nocentino shows how to use Powershell to distribute SSH keys to remote machines:

Folks in the Linux world are used to moving SSH keys to and from systems enabling password-less authentication using SSH user keys. Let’s take a minute to look at what it takes to use PowerShell to distribute SSH user keys to remote systems.

In the OpenSSH package there’s a command ssh-copy-id which is a bash script that copies a user’s public key to a remote systems. There’s a little intelligence in the script to set things up properly on the remote system for password-less key based authentication. If the appropriate directory and key file aren’t set up, ssh-copy-idwill create the directory and key file with the correct permissions on remote system. As far as I can tell, ssh-copy-id has not been implemented in the Win32-OpenSSH port. So that leaves us with implementing this functionality ourselves, in PowerShell.

Read on to see how he does it.

The Basics Of Bash: Writing Data

Mark Wilkinson hits us with some basic Bash output management:

If you have experience with PowerShell, some properties of Bash variables will feel familiar. In Bash, variables are denoted with a $ just like in PowerShell, but unlike PowerShell the $ is only needed when they are being referenced. When you are assigning a value to a variable, the $ is left off:

#!/bin/bashset -eset -umy_var="World"printf "Hello ${my_var}\n"

Above we assigned a value to my_var without using the $, but when we then referenced it in the printf statement, we had to use a $. We also enclosed the variable name in curly braces. This is not required in all cases, but it is a good idea to get in the habit of using them. In cases where you are using positional parameters above 9 (we’ll talk about this later) or you are using a variable in the middle of a string the braces are required, but there is no harm in adding them every time you use a variable in a string.

The basic syntax is pretty familiar to most programming languages, and there’s nothing scary about outputs, even when Mark starts getting into streams.

Backup And Restore With Move

John Morehouse has a script for database migration onto a differently configured server:

Now, not every environment is the same.  Instances get configured differently or things change just due to the nature of the business.  In a previous life I would routinely have to backup a database and restore it to another server.  However, the server I was using to restore to had a different drive configuration.  It happens.  Anyway, I wanted a script that would give me

  • A backup statement
  • A restore statement with the appropriate MOVE switch

This would allow me to easily execute the backup statement and then on the target server execute the restore statement.  Since the restore statement already had the appropriate MOVE switch provided, I didn’t have to manually compose the statement.

Click through for the script.

Module Signing In Practice

Solomon Rutzky shows how you can use certificate-based module signing to grant users fine-grained permissions:

-- 1) Create the Certificate:
CREATE CERTIFICATE [Permission$ViewServerState] ENCRYPTION BY PASSWORD = 'UseBetterPassword!' WITH SUBJECT = 'VIEW SERVER STATE permission', EXPIRY_DATE = '2099-12-31';
/*
The password is used to protect only the Private Key, not the Public
Key or the Certificate in general.
The default expiration date is 1 year from the date the Certificate is
created. However, in many situations, such as with Module Signing, the
expiration date is actually ignored. Still, I prefer to set the
expiration date to 2099.
*/

The DBA handling this server in the year 2100 is going to be really upset.  Certificate signing works wonders in conjunction with stored procedures, as you can grant procedures all kinds of rights needed for your application but users cannot run ad hoc queries expecting those same rights.

Dealing With Orphaned Users

Steve Jones uses dbatools to fix orphaned users:

One of the common issues that I would run into with refreshing development and test environments are the orphaned users. These are accounts that typically exist in production, but not in development. The logins and users are different in these environments, and often there isn’t a login on the development instance. This creates an orphaned user: one that exists in the database, but has no instance level mapping.

Cleaning up these users isn’t that hard, often with a removal of the user, mapping to a different login, or repairing this user by adding back the server login.  These aren’t difficult tasks, but the logic to write a script to quickly fix this, especially in a DR situation, may not be handy.

Read on to see how easy dbatools makes this.

Visualizing Types Of Joins

Kevin Feasel

2018-02-19

Syntax

Daniel Hutmacher has a nice PDF cheat sheet showing different types of joins:

How it works: For each join example, there are two tables, the left and the right table, shown as two columns. For the sake of simplicity, these tables are called “a” and “b” respectively in the code.

You’ll notice that the sheet uses a kind of pseudo-code when it comes to table names and column names.

Click through for the PDF.  I appreciate that he did not use Venn diagrams, as those should be saved for Union/Intersect/Except.

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