Views: Logical Data Independence

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From the course by University of Washington

Data Manipulation at Scale: Systems and Algorithms

680 ratings

University of Washington

Data Manipulation at Scale: Systems and Algorithms

680 ratings

Course 1 of 4 in the Specialization Data Science at Scale

Data analysis has replaced data acquisition as the bottleneck to evidence-based decision making --- we are drowning in it. Extracting knowledge from large, heterogeneous, and noisy datasets requires not only powerful computing resources, but the programming abstractions to use them effectively. The abstractions that emerged in the last decade blend ideas from parallel databases, distributed systems, and programming languages to create a new class of scalable data analytics platforms that form the foundation for data science at realistic scales. In this course, you will learn the landscape of relevant systems, the principles on which they rely, their tradeoffs, and how to evaluate their utility against your requirements. You will learn how practical systems were derived from the frontier of research in computer science and what systems are coming on the horizon. Cloud computing, SQL and NoSQL databases, MapReduce and the ecosystem it spawned, Spark and its contemporaries, and specialized systems for graphs and arrays will be covered. You will also learn the history and context of data science, the skills, challenges, and methodologies the term implies, and how to structure a data science project. At the end of this course, you will be able to: Learning Goals: 1. Describe common patterns, challenges, and approaches associated with data science projects, and what makes them different from projects in related fields. 2. Identify and use the programming models associated with scalable data manipulation, including relational algebra, mapreduce, and other data flow models. 3. Use database technology adapted for large-scale analytics, including the concepts driving parallel databases, parallel query processing, and in-database analytics 4. Evaluate key-value stores and NoSQL systems, describe their tradeoffs with comparable systems, the details of important examples in the space, and future trends. 5. “Think” in MapReduce to effectively write algorithms for systems including Hadoop and Spark. You will understand their limitations, design details, their relationship to databases, and their associated ecosystem of algorithms, extensions, and languages. write programs in Spark 6. Describe the landscape of specialized Big Data systems for graphs, arrays, and streams

From the lesson

Relational Databases and the Relational Algebra

Relational Databases are the workhouse of large-scale data management. Although originally motivated by problems in enterprise operations, they have proven remarkably capable for analytics as well. But most importantly, the principles underlying relational databases are universal in managing, manipulating, and analyzing data at scale. Even as the landscape of large-scale data systems has expanded dramatically in the last decade, relational models and languages have remained a unifying concept. For working with large-scale data, there is no more important programming model to learn.

Optimization: Physical Query Plans5:29

Optimization: Choosing Physical Plans4:54

Declarative Languages5:24

Declarative Languages: More Examples4:53

Views: Logical Data Independence5:07

Meet the Instructors

Bill Howe
Director of Research
Scalable Data Analytics

[MUSIC]

Okay, so we talked about physical data independence and

we talked about algebraic optimisation.

I wanna talk about another kind of data independence,

which is Logical Data Independence.

And so, we argued that physical data independence was this

ability to insulate applications and

protect applications from changes in the physical organization of the data.

All right?

So things were rearranged on disk,

we don't wanna have to rewrite all the code in the application.

And this is what databases provide and

relational databases in particular do a great job of providing this.

But if you go back to first paper, and even the quote I gave you,

he talks about insulating applications from the changes to

the internal representation, but also insulating applications

from changes to some forms of external representation.

And what he means by external is things like adding a column to a table.

So this isn't an internal shuffling of the bits on the disk.

It's actually a logic change to the table,

there's more data there than there was before.

But if you think about it if your code doesn't care about that new column you

shouldn't have to rewrite it just because there is a new column.

Okay.

So the ability to provide this logical data independence is provided by this

concept of views, and all relational databases have this concept.

Somewhat surprisingly, I find it to be somewhat underused in practice.

If you're using them right now, if you know what they are, great.

If you're using them, even better.

If you use databases but have never heard of views,

then this is a great time to learn about them.

Okay?

So, what is a view?

A view is just a query with a name.

So, I write a query, I give it a name and I put it in the database.

Now, I can then access that view as if it was a table

in the underlying database itself, as if it was a physical table.

So, why can we do this?

Well, I talked about this notion of algebraic closure before.

Right.

And this is exactly what it powers, what allows us to do this.

So we know that every query returns a table, right.

We take tables on the input.

We do some manipulation of them and we produce tables.

So we say that the language is algebraically closed.

And so any result of a view will always be something that we can then add

other queries on.

So we can stack queries on top of queries on top of queries on top of queries, okay.

So why might we want to do this?

So one reason is to protect the underlying data.

You can assign permissions to tables.

So for example, if you only want a particular user to see data associated

with their account, you can write a view that filters everything out except for

their account and then grant them access to that view.

The most direct benefit of this is that it allows you to expose data

according to a logical organization that makes sense for the user.

So even things as simple as hiding some join.

If you decide to reorganize your data into two tables requiring the programmers

use joins to link them back up again, you can simply write a view

that hides that join and let everyone access the result of it.

Now, maybe this may sound expensive but

the cool trick here is that because of this algebraic closure what happens is

the user's query gets composed with your query that defines the view and

the whole thing gets sent as one big block to the database for evaluation.

So the database simply doesn't care whether it came as a view and

then your user query or

whether it came all as one, all is one query directly from the programmer.

It's gonna optimize it the exact same way.

So there's nothing but a benefit here.

So let's see an example.

So given the schema purchase and product, define

a view called store price with two columns store and price, that has this definition.

So select store and select price from purchase and

product where the product IDs are equal.

[BLANK_AUDIO This

is a little funny because we didn't put pids here, so this is a little bit wrong.

So assume that each one of these has a, let's assume that this is pid and

assume that this is also pid and then it matches the query down here, okay?

So this result is now like a new table and

just like I said a second ago, you've now hidden the join from the users.

And so, complexities like these column names,

perhaps, you can insulate your users from and you can name them whatever you want.

And so, this allows you to put this, this is what this logical data independence

means, is that, no matter how I want to logically organize my tables,

I can expose a different perspective on the data than I wanna have myself.

And so this separates the people who are administering the data from the ones

you're actually accessing it, okay.

Logical data independence key idea.

[MUSIC]

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