CSE 544, Winter 2015, Homework 2

SimpleDB

Due: Monday, 9 February 2015, 11:45pm

 

Acknowledgement

This assignment comes from Prof. Sam Madden's 6.830 class at MIT.

The full series of SimpleDB assignments includes what we will do in this lab, which is to build the basic functionality for query processing. It also includes transactions and query optimization, which we will NOT do.

We also use this series of assignments in 444. We have contributed bug fixes and an extra lab to SimpleDB. The lab that we added involves building a parallel, shared-nothing version of SimpleDB. We invite you to take a look at the CSE 444 course webpage to see what all the SimpleDB labs are about.

 

Homework 2 Clarifications

Please let us know if you encounter any problems with this assignment. We will post clarifications and fixes here as necessary. As with HW1, please let us know if this assignment takes an unusually large amount of time.

 

Assignment goal

In this assignment, you will write a basic database management system called SimpleDB. First, you will implement the core modules required to access stored data on disk. You will then write a set of operators for SimpleDB to implement selections, joins, and aggregates. The end result is a database system that can perform simple queries over multiple tables. We will not ask you to add transactions, locking, and concurrent queries because we do not have time to do the full project in 544. However, we invite you to think how you would add such functionality into the system.

SimpleDB is written in Java. We have provided you with a set of mostly unimplemented classes and interfaces. You will need to write the code for these classes. We will grade your code by running a set of system tests written using JUnit. We have also provided a number of unit tests, which we will not use for grading but that you may find useful in verifying that your code works. Note that the unit tests we provide are to help guide your implementation along, but they are not intended to be comprehensive or to establish correctness.

The remainder of this document describes the basic architecture of SimpleDB, gives some suggestions about how to start coding, and discusses how to hand in your assignment.

We strongly recommend that you start as early as possible on this assignment. It requires you to write a fair amount of code!

 

0. Find bugs, be patient, earn candy bars

SimpleDB is a relatively complex piece of code. It is very possible you are going to find bugs, inconsistencies, and bad, outdated, or incorrect documentation, etc.

We ask you, therefore, to do this assignment with an adventurous mindset. Don't get mad if something is not clear, or even wrong; rather, try to figure it out yourself or send us a friendly email. We promise to help out by posting bug fixes, new tarballs, etc., as bugs and issues are reported.


1. Getting started

These instructions are written for any Unix-based platform (e.g., Linux, MacOS, etc.). Because the code is written in Java, it should work under Windows as well, though the directions in this document may not apply.

We have included Section 1.2 on using the project with Eclipse. Using Eclipse is recommended, especially if you are on Windows.

Download the code from here and untar it. In linux:

$ wget http://www.cs.washington.edu/education/courses/cse544/15wi/hw/hw2/hw2.tar.gz 
$ tar zxvf hw2.tar.gz
$ cd hw2

In Windows, use a program like 7zip (extract hw2.tar.gz using 7zip to get hw2.tar, then extract hw2.tar with 7zip to unpack it into a directory).

SimpleDB uses the Ant build tool to compile the code and run tests. Ant is similar to make, but the build file is written in XML and is somewhat better suited to Java code. Most modern Linux distributions include Ant.

To help you during development, we have provided a set of unit tests in addition to the end-to-end tests that we use for grading. These are by no means comprehensive, and you should not rely on them exclusively to verify the correctness of your project.

To run the unit tests use the test build target:

$ cd hw2
$ # run all unit tests
$ ant test
$ # run a specific unit test
$ ant runtest -Dtest=TupleTest

You should see output similar to:

# build output...

test:
  [junit] Running simpledb.TupleTest
  [junit] Testsuite: simpledb.TupleTest
  [junit] Tests run: 3, Failures: 0, Errors: 3, Time elapsed: 0.036 sec
  [junit] Tests run: 3, Failures: 0, Errors: 3, Time elapsed: 0.036 sec
  
# ... stack traces and error reports ...

The output above indicates that three errors occurred during compilation; this is because the code we have given you doesn't yet work. As you complete parts of the assignment, you will work towards passing additional unit tests. If you wish to write new unit tests as you code, they should be added to the test/simpledb directory.

For more details about how to use Ant, see the manual. The Running Ant section provides details about using the ant command. However, the quick reference table below should be sufficient for working on the assignments.

CommandDescription
antBuild the default target (for simpledb, this is dist).
ant -projecthelpList all the targets in build.xml with descriptions.
ant distCompile the code in src and package it in dist/simpledb.jar.
ant testCompile and run all the unit tests.
ant runtest -Dtest=testnameRun the unit test named testname.
ant systemtestCompile and run all the system tests.
ant runsystest -Dtest=testnameCompile and run the system test named testname.


1.1. Running end-to-end tests

We have also provided a set of end-to-end tests that will eventually be used for grading. These tests are structured as JUnit tests that live in the test/simpledb/systemtest directory. To run all the system tests, use the systemtest build target:

 

  
$ ant systemtest

# ... build output ...

systemtest:

[junit] Running simpledb.systemtest.ScanTest
  [junit] Testsuite: simpledb.systemtest.ScanTest
  [junit] Tests run: 3, Failures: 0, Errors: 3, Time elapsed: 0.237 sec
  [junit] Tests run: 3, Failures: 0, Errors: 3, Time elapsed: 0.237 sec
  [junit] 
  [junit] Testcase: testSmall took 0.017 sec
  [junit] 	Caused an ERROR
  [junit] implement this
  [junit] java.lang.UnsupportedOperationException: implement this
  [junit] 	at simpledb.HeapFile.id(HeapFile.java:46)
  [junit] 	at simpledb.systemtest.SystemTestUtil.matchTuples(SystemTestUtil.java:90)
  [junit] 	at simpledb.systemtest.SystemTestUtil.matchTuples(SystemTestUtil.java:83)
  [junit] 	at simpledb.systemtest.ScanTest.validateScan(ScanTest.java:30)
  [junit] 	at simpledb.systemtest.ScanTest.testSmall(ScanTest.java:41)
    
# ... more error messages ...

This indicates that this test failed, showing the stack trace where the error was detected. To debug, start by reading the source code where the error occurred. When the tests pass, you will see something like the following:

$ ant systemtest

# ... build output ...

    [junit] Testsuite: simpledb.systemtest.ScanTest
    [junit] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 7.278 sec
    [junit] Tests run: 3, Failures: 0, Errors: 0, Time elapsed: 7.278 sec
    [junit] 
    [junit] Testcase: testSmall took 0.937 sec
    [junit] Testcase: testLarge took 5.276 sec
    [junit] Testcase: testRandom took 1.049 sec

BUILD SUCCESSFUL
Total time: 52 seconds

 

1.1.1 Creating dummy tables


It is likely you'll want to create your own tests and your own data tables to test your own implementation of SimpleDB. You can create any .txt file and convert it to a .dat file in SimpleDB's HeapFile format using the command:

$ ant dist

$ java -jar dist/simpledb.jar convert file.txt N

where file.txt is the name of the file and N is the number of columns in the file. Notice that file.txt has to be in the following format:

int1,int2,...,intN
int1,int2,...,intN
int1,int2,...,intN int1,int2,...,intN

...where each intN is a non-negative integer.

To view the contents of a table, use the print command. Note that this command will not work until later in the assignment:

$ java -jar dist/simpledb.jar print file.dat N

where file.dat is the name of a table created with the convert command, and N is the number of columns in the file.


1.2. Working in Eclipse

Eclipse is a graphical software development environment that you might be more comfortable with working in. The instructions we provide were generated by using Eclipse Juno for Java Developers (not the enterprise edition) with Java 1.7.0_09 on OS X Yosemite, but other versions should also work under Windows or on Linux.

Setting the Assignment Up in Eclipse

Running Individual Unit and System Tests

To run a unit test or system test (both are JUnit tests, and can be initialized the same way), go to the Package Explorer tab on the left side of your screen. Under the "hw2" project, open the "test" directory. Unit tests are found in the "simpledb" package, and system tests are found in the "simpledb.systemtests" package. To run one of these tests, select the test (they are all called *Test.java - don't select TestUtil.java or SystemTestUtil.java), right click on it, select "Run As," and select "JUnit Test." This will bring up a JUnit tab, which will tell you the status of the individual tests within the JUnit test suite, and will show you exceptions and other errors that will help you debug problems.

Running Ant Build Targets

If you want to run commands such as "ant test" or "ant systemtest," right click on build.xml in the Package Explorer. Select "Run As" and then "Ant Build..." (note: select the option with the ellipsis (...), otherwise you won't be presented with a set of build targets to run). Then, in the "Targets" tab of the next screen, check off the targets you want to run (probably "dist" and one of "test" or "systemtest"). This should run the build targets and show you the results in Eclipse's console window.


1.3. Implementation hints

Before beginning to write code, we strongly encourage you to read through this entire document to get a feel for the high-level design of SimpleDB.

You will need to fill in any piece of code that is not implemented. It will be obvious where we think you should write code. You may need to add private methods and/or helper classes. You may change APIs, but make sure our grading tests still run and make sure to mention, explain, and defend your decisions in your writeup.

In addition to the methods that you need to fill out for this assignment, the class interfaces contain numerous methods that you need not implement in this assignment. These will either be indicated per class:

// Not necessary for this assignment
public class Insert implements DbIterator {

or per method:

public boolean deleteTuple(Tuple t) throws DbException {

  // Not necessary for this assignment
  return false;
}

The code that you submit should compile without having to modify these methods.

We suggest exercises along this document to guide your implementation, but you may find that a different order makes more sense for you. Here's a rough outline of one way you might proceed with your SimpleDB implementation:

At this point you should be able to pass all of the tests in the ant systemtest target, which is the goal of this lab. Section 2 below walks you through these implementation steps and the unit tests corresponding to each one in more detail.

 

1.4. Transactions, locking, and recovery

As you look through the interfaces that we have provided you, you will see a number of references to locking, transactions, and recovery. You do not need to support these features. We will not be implementing this part of SimpleDB in 544. The test code we have provided you with generates a fake transaction ID that is passed into the operators of the query it runs; you should pass this transaction ID into other operators and the buffer pool.

2. SimpleDB Architecture and Implementation Guide

SimpleDB consists of:

SimpleDB does not include many things that you may think of as being a part of a "database." In particular, SimpleDB does not have:

In the rest of this Section, we describe each of the main components of SimpleDB that you will need to implement in this lab. You should use the exercises in this discussion to guide your implementation. This document is by no means a complete specification for SimpleDB; you will need to make decisions about how to design and implement various parts of the system. Note that for Lab 1 you do not need to implement any operators (e.g., select, join, project) except sequential scan. You will add support for additional operators in future labs.

2.1. The Database Class

The Database class provides access to a collection of static objects that are the global state of the database. In particular, this includes methods to access the catalog (the list of all the tables in the database), the buffer pool (the collection of database file pages that are currently resident in memory), and the log file. You will not need to worry about the log file in this lab. We have implemented the Database class for you. You should take a look at this file as you will need to access these objects.

2.2. Fields and Tuples

Tuples in SimpleDB are quite basic. They consist of a collection of Field objects, one per field in the Tuple. Field is an interface that different data types (e.g., integer, string) implement. Tuple objects are created by the underlying access methods (e.g., heap files, or B-trees), as described in the next section. Tuples also have a type (or schema), called a tuple descriptor, represented by a TupleDesc object. This object consists of a collection of Type objects, one per field in the tuple, each of which describes the type of the corresponding field.

Exercise 1. Implement the skeleton methods in: At this point, your code should pass the unit tests TupleTest and TupleDescTest. At this point, modifyRecordId() should fail because you havn't implemented it yet.

 

2.3. Catalog

The catalog (class Catalog in SimpleDB) consists of a list of the tables and schemas of the tables that are currently in the database. You will need to support the ability to add a new table, as well as getting information about a particular table. Associated with each table is a TupleDesc object that allows operators to determine the types and number of fields in a table.

The global catalog is a single instance of Catalog that is allocated for the entire SimpleDB process. The global catalog can be retrieved via the method Database.getCatalog(), and the same goes for the global buffer pool (using Database.getBufferPool()).

Exercise 2. Implement the skeleton methods in:

At this point, your code should pass the unit tests in CatalogTest.

2.4. BufferPool

The buffer pool (class BufferPool in SimpleDB) is responsible for caching pages in memory that have been recently read from disk. All operators read and write pages from various files on disk through the buffer pool. It consists of a fixed number of pages, defined by the numPages parameter to the BufferPool constructor. In later labs, you will implement an eviction policy. For this lab, you only need to implement the constructor and the BufferPool.getPage() method used by the SeqScan operator. The BufferPool should store up to numPages pages. For this lab, if more than numPages requests are made for different pages, then instead of implementing an eviction policy, you may throw a DbException. In future labs you will be required to implement an eviction policy.

The Database class provides a static method, Database.getBufferPool(), that returns a reference to the single BufferPool instance for the entire SimpleDB process.

Exercise 3. Implement the getPage() method in: We have not provided unit tests for BufferPool. The functionality you implemented will be tested in the implementation of HeapFile below. You should use the DbFile.readPage method to access pages of a DbFile.

2.5. HeapFile access method

Access methods provide a way to read or write data from disk that is arranged in a specific way. Common access methods include heap files (unsorted files of tuples) and B-trees; for this assignment, you will only implement a heap file access method, and we have written some of the code for you.

A HeapFile object is arranged into a set of pages, each of which consists of a fixed number of bytes for storing tuples, (defined by the constant BufferPool.PAGE_SIZE), including a header. In SimpleDB, there is one HeapFile object for each table in the database. Each page in a HeapFile is arranged as a set of slots, each of which can hold one tuple (tuples for a given table in SimpleDB are all of the same size). In addition to these slots, each page has a header that consists of a bitmap with one bit per tuple slot. If the bit corresponding to a particular tuple is 1, it indicates that the tuple is valid; if it is 0, the tuple is invalid (e.g., has been deleted or was never initialized.) Pages of HeapFile objects are of type HeapPage which implements the Page interface. Pages are stored in the buffer pool but are read and written by the HeapFile class.

SimpleDB stores heap files on disk in more or less the same format they are stored in memory. Each file consists of page data arranged consecutively on disk. Each page consists of one or more bytes representing the header, followed by the BufferPool.PAGE_SIZE - # header bytes bytes of actual page content. Each tuple requires tuple size * 8 bits for its content and 1 bit for the header. Thus, the number of tuples that can fit in a single page is:

tupsPerPage = floor((BufferPool.PAGE_SIZE * 8) / (tuple size * 8 + 1))

Where tuple size is the size of a tuple in the page in bytes. The idea here is that each tuple requires one additional bit of storage in the header. We compute the number of bits in a page (by mulitplying PAGE_SIZE by 8), and divide this quantity by the number of bits in a tuple (including this extra header bit) to get the number of tuples per page. The floor operation rounds down to the nearest integer number of tuples (we don't want to store partial tuples on a page!)

Once we know the number of tuples per page, the number of bytes required to store the header is simply:

headerBytes = ceiling(tupsPerPage/8)

The ceiling operation rounds up to the nearest integer number of bytes (we never store less than a full byte of header information.)

The low (least significant) bits of each byte represents the status of the slots that are earlier in the file. Hence, the lowest bit of the first byte represents whether or not the first slot in the page is in use. Also, note that the high-order bits of the last byte may not correspond to a slot that is actually in the file, since the number of slots may not be a multiple of 8. Also note that all Java virtual machines are big-endian.

Exercise 4. Implement the skeleton methods in:

Although you will not use them directly in Lab 1, we ask you to implement getNumEmptySlots() and isSlotUsed() in HeapPage. These require pushing around bits in the page header. You may find it helpful to look at the other methods that have been provided in HeapPage or in src/java/simpledb/HeapFileEncoder.java to understand the layout of pages.

You will also need to implement an Iterator over the tuples in the page, which may involve an auxiliary class or data structure.

At this point, your code should pass the unit tests in HeapPageIdTest, RecordIdTest, and HeapPageReadTest.

After you have implemented HeapPage, you will write methods for HeapFile in this lab to calculate the number of pages in a file and to read a page from the file. You will then be able to fetch tuples from a file stored on disk.

Exercise 5. Implement the skeleton methods in:

To read a page from disk, you will first need to calculate the correct offset in the file. Hint: you will need random access to the file in order to read and write pages at arbitrary offsets. You should not call BufferPool methods when reading a page from disk.

You will also need to implement the HeapFile.iterator() method, which should iterate through through the tuples of each page in the HeapFile. The iterator must use the BufferPool.getPage() method to access pages in the HeapFile. This method loads the page into the buffer pool and will eventually be used (in a later lab) to implement locking-based concurrency control and recovery. Do not load the entire table into memory on the open() call -- this will cause an out of memory error for very large tables.

At this point, your code should pass the unit tests in HeapFileReadTest.

2.6. Operators

Operators are responsible for the actual execution of the query plan. They implement the operations of the relational algebra. In SimpleDB, operators are iterator based; each operator implements the DbIterator interface.

Operators are connected together into a plan by passing lower-level operators into the constructors of higher-level operators, i.e., by 'chaining them together.' Special access method operators at the leaves of the plan are responsible for reading data from the disk (and hence do not have any operators below them).

At the top of the plan, the program interacting with SimpleDB simply calls getNext on the root operator; this operator then calls getNext on its children, and so on, until these leaf operators are called. They fetch tuples from disk and pass them up the tree (as return arguments to getNext); tuples propagate up the plan in this way until they are output at the root or combined or rejected by another operator in the plan.

2.6.1. Scan

Exercise 6. Implement the skeleton methods in:

This operator sequentially scans all of the tuples from the pages of the table specified by the tableid in the constructor. This operator should access tuples through the DbFile.iterator() method.

At this point, you should be able to complete the ScanTest system test. Good work!

2.6.2. Filter and Join

Recall that SimpleDB DbIterator classes implement the operations of the relational algebra. You will now implement two operators that will enable you to perform queries that are slightly more interesting than a table scan.
Exercise 7. Implement the skeleton methods in: At this point, your code should pass the unit tests in PredicateTest, JoinPredicateTest, FilterTest, and JoinTest. Furthermore, you should be able to pass the system tests FilterTest and JoinTest.

2.6.3. Aggregates (EXTRA CREDIT)

All the materials in this section is optional and will count only as extra credit.

An additional SimpleDB operator implements basic SQL aggregates with a GROUP BY clause. You should implement the five SQL aggregates (COUNT, SUM, AVG, MIN, MAX) and support grouping. You only need to support aggregates over a single field, and grouping by a single field.

In order to calculate aggregates, we use an Aggregator interface which merges a new tuple into the existing calculation of an aggregate. The Aggregator is told during construction what operation it should use for aggregation. Subsequently, the client code should call Aggregator.mergeTupleIntoGroup() for every tuple in the child iterator. After all tuples have been merged, the client can retrieve a DbIterator of aggregation results. Each tuple in the result is a pair of the form (groupValue, aggregateValue), unless the value of the group by field was Aggregator.NO_GROUPING, in which case the result is a single tuple of the form (aggregateValue).

Note that this implementation requires space linear in the number of distinct groups. For the purposes of this lab, you do not need to worry about the situation where the number of groups exceeds available memory.

Exercise 8. Implement the skeleton methods in: At this point, your code should pass the unit tests IntegerAggregatorTest, StringAggregatorTest, and AggregateTest. Furthermore, you should be able to pass the AggregateTest system test.

2.7. Query Parser and Contest (EXTRA CREDIT)

All the materials in this section is optional and will count only as extra credit.

We've provided you with a query parser for SimpleDB that you can use to write and run SQL queries against your database once you have completed the exercises in this lab.

The first step is to create some data tables and a catalog. Suppose you have a file data.txt with the following contents:

1,10
2,20
3,30
4,40
5,50
5,50
You can convert this into a SimpleDB table using the convert command (make sure to type ant first!):
java -jar dist/simpledb.jar convert data.txt 2 "int,int"
This creates a file data.dat. In addition to the table's raw data, the two additional parameters specify that each record has two fields and that their types are int and int.

Next, create a catalog file, catalog.txt, with the follow contents:

data (f1 int, f2 int)
This tells SimpleDB that there is one table, data (stored in data.dat) with two integer fields named f1 and f2.

Finally, invoke the parser. You must run java from the command line (ant doesn't work properly with interactive targets.) From the simpledb/ directory, type:

java -jar dist/simpledb.jar parser catalog.txt
You should see output like:
Added table : data with schema INT(f1), INT(f2), 
SimpleDB> 
Finally, you can run a query:
SimpleDB> select d.f1, d.f2 from data d;
Started a new transaction tid = 1221852405823
 ADDING TABLE d(data) TO tableMap
     TABLE HAS  tupleDesc INT(d.f1), INT(d.f2), 
1       10
2       20
3       30
4       40
5       50
5       50

 6 rows.
----------------
0.16 seconds

SimpleDB> 
The parser is relatively full featured (including support for SELECTs, INSERTs, DELETEs, and transactions), but does have some problems and does not necessarily report completely informative error messages. Here are some limitations to bear in mind:

Exercise 9: Please execute the three queries below using your SimpleDB prototype and report the times in your lab write-up.

We have built a SimpleDB-encoded version of the DBLP database; the needed files are located at: http://www.cs.washington.edu/education/courses/cse544/15wi/hw/hw2/dblp_data.tar.gz

You should download the file and unpack it. It will create four files in the dblp_data directory. Move them into the simpledb directory. The following commands will acomplish this, if you run them from the simpledb directory:

wget http://www.cs.washington.edu/education/courses/cse544/15wi/hw/hw2/dblp_data.tar.gz
tar xvzf dblp_data.tar.gz
mv dblp_data/* .
rm -r dblp_data.tar.gz dblp_data

You can then run the parser with:

java -jar dist/simpledb.jar parser dblp_simpledb.schema

We will start a thread on the course message board inviting anyone interested to post their runtimes for the following three queries (please run the queries on a lab machine and indicate which one you used so it becomes easier to compare runtimes). The contest is just for fun. It will not affect your grade:

  1. SELECT p.title
    FROM papers p
    WHERE p.title LIKE 'selectivity';
    
  2. SELECT p.title, v.name
    FROM papers p, authors a, paperauths pa, venues v
    WHERE a.name = 'E. F. Codd'
    AND pa.authorid = a.id
    AND pa.paperid = p.id
    AND p.venueid = v.id;
     
  3. SELECT a2.name, count(p.id)
    FROM papers p, authors a1, authors a2, paperauths pa1, paperauths pa2
    WHERE a1.name = 'Michael Stonebraker'
    AND pa1.authorid = a1.id 
    AND pa1.paperid = p.id 
    AND pa2.authorid = a2.id 
    AND pa1.paperid = pa2.paperid
    GROUP BY a2.name
    ORDER BY a2.name;
     
    

Note that each query will print out its runtime after it executes.

You may wish to create optimized implementations of some of the operators; in particular, a fast join operator (e.g., not nested loops) will be essential for good performance on queries 2 and 3.

There is currently no optimizer in the parser, so the queries above have been written to cause the parser to generate reasonable plans. Here are some helpful hints about how the parser works that you may wish to exploit while running these queries:

Our reference implementation can run Query 1 in about .35 seconds, Query 2 in about 10 seconds, and Query 3 in about 20 seconds. We implemented a special-purpose join operator for equality joins but did little else to optimize performance.

Depending on the efficiency of your implementation, each of these queries will take seconds to minutes to run to completion, outputting tuples as they are computed. Certainly don't expect the level of performance of postgres. :)


Turn in instructions

You must submit your code (see below) as well as a short (2 pages, maximum) writeup describing your approach. This writeup should:

To submit your code, please create a hw2.tar.gz tarball (such that, untarred, it creates a hw2/src/simpledb directory with your code) and submit it to Catalyst: https://catalyst.uw.edu/collectit/dropbox/mbalazin/33981

You may submit your code multiple times; we will use the latest version you submit that arrives before the deadline (before 11:45 PM on the due date). Please also include your writeup as a PDF or text file in the submission package.

Submitting a bug

Please submit (friendly!) bug reports to the TA and instructor. When you do, please try to include:


Grading

50% of your grade will be based on whether or not your code passes the system test suite we will run over it. These tests will be a superset of the tests we have provided. Before handing in your code, you should make sure it produces no errors (passes all of the tests) from both ant test and ant systemtest.

Important: before testing, we will replace your build.xml, HeapFileEncoder.java, and the entire contents of the test directory with our version of these files. This means you cannot change the format of .dat files! You should also be careful changing our APIs. You should test that your code compiles the unmodified tests. In other words, we will untar your tarball, replace the files mentioned above, compile it, and then grade it. It will look roughly like this:

$ tar xvzf hw2.tar.gz
$ cd ./hw2
[replace build.xml, HeapFileEncoder.java, and test]
$ ant test
$ ant systemtest
[additional tests]

An additional 50% of your grade will be based on the quality of your writeup and our subjective evaluation of your code.

Extra credit: 5% for each.

We hope you will enjoy this assignment and will learn a lot about how a simple DBMS system can be implemented!