CS133 Lab 2: SimpleDB Operators


Lab Description

In this lab assignment, you will write a set of operators for SimpleDB to implement table modifications (e.g., insert and delete records), selections, joins, and aggregates. These will build on top of the foundation that you wrote in Lab 1 to provide you with a database system that can perform simple queries over multiple tables.

Additionally, we ignored the issue of buffer pool management in Lab 1: we have not dealt with the problem that arises when we reference more pages than we can fit in memory over the lifetime of the database. Now in Lab 2, you will design an eviction policy to flush stale pages from the buffer pool.

You do not need to implement transactions or locking in this lab.

The remainder of this document gives some suggestions about how to start coding, describes a set of exercises to help you work through the lab, and discusses how to hand in your code. This lab requires you to write a fair amount of code, so we encourage you to start early!

Quick jump to exercises:

1. Getting started

You should begin with the code you submitted for Lab 1 (if you did not submit code for Lab 1, or your solution didn't work properly, contact me to discuss options). We have provided you with extra code and extra test cases for this lab that are not in the original code distribution you received. We reiterate 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.

You will need to add these new files to your release. The easiest way to do this is to untar the new code in the same directory as your top-level simpledb directory, as follows:

Now all files from lab1 and lab2 will be in the cs133-lab2 directory.

To work in Eclipse, create a new java project named cs133-lab2 like you did for lab1. You may need one more step to compile:

Right-click the project name (likely cs133-lab2) in the Package Explorer and select Properties. Choose Java Build Path on the left-hand-side, and click on the Libraries tab. Push the Add JARs... button, select zql.jar and jline-0.9.94.jar, and push OK, followed by apply and close. Your code should now compile.

If you used the Ant window in Eclipse for Lab 1: You'll want to remove that build file from the window and add the one for Lab 2.

1.1. Implementation Notes

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

We will grade your assignment by looking at your code and verifying that you have passed the test for the ant targets test and systemtest. See Section 3.3 for a complete discussion of grading and list of the tests you will need to pass.

Here's an overview of your SimpleDB implementation for this lab; more details on the steps in this outline, including exercises, are given in Section 2.

  1. Implement the operators Filter and Join and verify that their corresponding tests work. The Javadoc comments for these operators contain details about how they should work. We have given you implementations of Project and OrderBy which may help you understand how other operators work.

  2. Implement IntegerAggregator and StringAggregator. Here, you will write the logic that actually computes an aggregate over a particular field across multiple groups in a sequence of input tuples. Use integer division for computing the average, since SimpleDB only supports integers. StringAggegator only needs to support the COUNT aggregate, since the other operations do not make sense for strings.

  3. Implement the Aggregate operator. As with other operators, aggregates implement the DbIterator interface so that they can be placed in SimpleDB query plans. Note that the output of an Aggregate operator is an aggregate value of an entire group for each call to next(), and that the aggregate constructor takes the aggregation and grouping fields.

  4. Implement the methods related to tuple insertion, deletion, and page eviction in BufferPool. You do not need to worry about transactions at this point.

  5. Implement the Insert and Delete operators. Like all operators, Insert and Delete implement DbIterator, accepting a stream of tuples to insert or delete and outputting a single tuple with an integer field that indicates the number of tuples inserted or deleted. These operators will need to call the appropriate methods in BufferPool that actually modify the pages on disk. Check that the tests for inserting and deleting tuples work properly.

    Note that SimpleDB does not implement any kind of consistency or integrity checking, so it is possible to insert duplicate records into a file and there is no way to enforce primary or foreign key constraints.

By the end of this lab, you'll also be able to use the provided SQL parser to run SQL queries against your database! See Section 2.7.

The Operator class

Finally, you might notice that the iterators in this lab extend the abstract class Operator instead of implementing the DbIterator interface. Because the implementation of next/hasNext is often repetitive, annoying, and error-prone, Operator implements this logic generically, and only requires that you implement a simpler fetchNext. Feel free to use this style of implementation, or just implement the DbIterator interface if you prefer, as you did with SeqScan in Lab 1. To implement the DbIterator interface, remove extends Operator from iterator classes, and in its place put implements DbIterator. If you plan to use Operator.java, be sure to browse the code and comments to see what is going on and which methods need to be implemented for the classes that extend it. You can look at Project.java to see examples of how you should call super in your operators.

2. SimpleDB Architecture and Implementation Guide

2.1. 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 1. 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.

Some helpful notes:

2.2. Aggregates

An additional SimpleDB operator implements basic SQL aggregates with a GROUP BY clause. Now you will 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. You can ignore the other aggregate functions that you see in the Aggregator class.

In order to calculate aggregates, the Aggregate operator uses a helper interface called Aggregator which does the work of merging the next tuple into the existing calculation of an aggregate. Depending on the type of the field being aggregated (Type.INT_TYPE or Type.STRING_TYPE), you should create an IntegerAggregator or a StringAggregator. The Aggregator is told during construction which operation it should use for aggregation (see Aggregator.Op). Subsequently, the Aggregate operator should call Aggregator.mergeTupleIntoGroup() for every tuple in its child iterator. After all tuples have been merged, the Aggregate operator can retrieve a DbIterator of aggregation results from the Aggregator. Each tuple in the result should be 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. However, you should not assume all tuples can fit in memory. You should not store lists of tuples in Aggregators, but instead store "running" aggregates. For example, for SUM you could update the "sum so far" as tuples are merged in.
Note: for AVG, you should keep a running sum and running count, and perform the division just once at the end of merging all tuples.

Exercise 2. 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.

Some helpful notes:

2.3. HeapFile Mutability

Now, we will begin to implement methods to support modifying tables. If you haven't yet already read through Section 2.4, you may find it helpful to do so now. You will see that you will eventually write Insert and Delete operators. These operators call the insert and delete methods from BufferPool, respectively, which in turn calls the appropriate methods on a HeapFile. Then the HeapFile calls insert or delete on the correct HeapPage!

We begin implementing Insert and Delete at the level of individual pages and files. There are two main sets of operations: adding tuples and removing tuples.

Removing tuples: To remove a tuple, you will need to implement deleteTuple. Tuples contain RecordIDs which allow you to find the page they reside on, so this should be as simple as locating the page a tuple belongs to and modifying the header of the page appropriately. Note that this simplicity assumes that the header is consulted when retrieving tuples or deciding where to insert a new tuple.

Adding tuples: The insertTuple method in HeapFile.java is responsible for adding a tuple to a heap file. To add a new tuple to a HeapFile, you will have to find a page with an empty slot. If no such pages exist in the HeapFile, you need to create a new page and append it to the physical file on disk (use the static method HeapPage.createEmptyPageData()), but after writing to it disk be sure to get the page via the Buffer Pool before inserting the tuple. You will also need to ensure that the RecordId in the tuple is updated correctly.

Note that it is important that the HeapFile.insertTuple() and HeapFile.deleteTuple() methods access pages using the BufferPool.getPage() method; otherwise, your implementation of transactions and locking in a future lab will not work properly.

Exercise 3. Implement the remaining skeleton methods in:

Next you'll finish the insertion/deletion in the BufferPool. These methods should call the appropriate methods in the HeapFile that belong to the table being modified (this extra level of indirection is needed to support other types of files that could exist, such as indexes. Note that the HeapFile insert/delete methods should return which pages were dirtied so that the BufferPool can call markDirty on those pages.

Implement the following skeleton methods in src/simpledb/BufferPool.java:

At this point, your code should pass the unit tests in HeapPageWriteTest, HeapFileWriteTest, and BufferPoolWriteTest.

Some helpful notes:

2.4. Insertion and deletion

Now that you have written all of the HeapFile machinery to add and remove tuples, you will implement the Insert and Delete operators.

For plans that implement insert and delete queries, the top-most operator is a special Insert or Delete operator that modifies the pages on disk. For their next() calls, these operators return the number of affected tuples. This is implemented by returning a single tuple with one integer field, containing the count.

Exercise 4. Implement the skeleton methods in: At this point, your code should pass the unit tests in InsertTest. We have not provided unit tests for Delete. Furthermore, you should be able to pass the InsertTest and DeleteTest system tests.

Some helpful notes:

2.5. Page eviction

In Lab 1, we did not correctly observe the limit on the maximum number of pages in the buffer pool defined by the constructor argument numPages. Now, you will choose a page eviction policy and instrument any previous code that reads or creates pages to implement your policy.

When more than numPages pages are in the buffer pool, one page should be evicted from the pool before the next is loaded. The choice of eviction policy is up to you; it is not necessary to do something more sophisticated than discussed in class or even a random policy. Describe your policy in the lab writeup.

Notice that BufferPool asks you to implement a flushAllPages() method. This is not something you would ever need in a real implementation of a buffer pool. However, we need this method for testing purposes. You should never call this method from any non-testing code. Because of the way we have implemented ScanTest.cacheTest, you will need to ensure that your flushPage and flushAllPages methods do not evict pages from the buffer pool to properly pass this test. flushAllPages should call flushPage on all pages in the BufferPool, and flushPage should write any dirty page to disk and mark it as not dirty, while leaving it in the BufferPool. The only method which should remove a page from the buffer pool is evictPage, which should call flushPage on any dirty page it evicts.

Exercise 5. Fill in the methods flushPage(), flushAllPages(), evictPage(), as well as any updates to getPage() to implement page eviction in:

If you did not implement writePage() in HeapFile.java above, you will also need to do that now.

At this point, your code should pass the EvictionTest system test.

Since we will not be checking for any particular eviction policy, this test works by creating a BufferPool with 16 pages (NOTE: while DEFAULT_PAGES is 50, we are initializing the BufferPool with less!), scanning a file with many more than 16 pages, and seeing if the memory usage of the JVM increases by more than 5 MB. If you do not implement an eviction policy correctly, you will not evict enough pages, and will go over the size limitation, thus failing the test.

Some helpful notes:

You have now completed the code for this lab. Good work!

2.6. Query walkthrough

The following code implements a simple join query between two tables, each consisting of three columns of integers. (The file some_data_file1.dat and some_data_file2.dat are binary representation of the pages from this file). This code is equivalent to the SQL statement:

  FROM some_data_file1, some_data_file2 
  WHERE some_data_file1.field1 = some_data_file2.field1
  AND some_data_file1.id > 1
For more extensive examples of query operations, you may find it helpful to browse the unit tests for joins, filters, and aggregates.
package simpledb;
import java.io.*;

public class jointest {

    public static void main(String[] argv) {
        // construct a 3-column table schema
        Type types[] = new Type[]{ Type.INT_TYPE, Type.INT_TYPE, Type.INT_TYPE };
        String names[] = new String[]{ "field0", "field1", "field2" };

        TupleDesc td = new TupleDesc(types, names);

        // create the tables, associate them with the data files
        // and tell the catalog about the schema  the tables.
        HeapFile table1 = new HeapFile(new File("some_data_file1.dat"), td);
        Database.getCatalog().addTable(table1, "t1");

        HeapFile table2 = new HeapFile(new File("some_data_file2.dat"), td);
        Database.getCatalog().addTable(table2, "t2");

        // construct the query: we use two SeqScans, which spoonfeed
        // tuples via iterators into join
        TransactionId tid = new TransactionId();

        SeqScan ss1 = new SeqScan(tid, table1.getId(), "t1");
        SeqScan ss2 = new SeqScan(tid, table2.getId(), "t2");

        // create a filter for the where condition
        Filter sf1 = new Filter(
                                new Predicate(0,
                                Predicate.Op.GREATER_THAN, new IntField(1)),  ss1);

        JoinPredicate p = new JoinPredicate(1, Predicate.Op.EQUALS, 1);
        Join j = new Join(p, sf1, ss2);

        // and run it
        try {
            while (j.hasNext()) {
                Tuple tup = j.next();

        } catch (Exception e) {



Both tables have three integer fields. To express this, we create a TupleDesc object and pass it an array of Type objects indicating field types and String objects indicating field names. Once we have created this TupleDesc, we initialize two HeapFile objects representing the tables. Once we have created the tables, we add them to the Catalog. (If this were a database server that was already running, we would have this catalog information loaded; we need to load this only for the purposes of this test).

Once we have finished initializing the database system, we create a query plan. Our plan consists of two SeqScan operators that scan the tuples from each file on disk, connected to a Filter operator on the first HeapFile, connected to a Join operator that joins the tuples in the tables according to the JoinPredicate. In general, these operators are instantiated with references to the appropriate table (in the case of SeqScan) or child operator (in the case of e.g., Join). The test program then repeatedly calls next on the Join operator, which in turn pulls tuples from its children. As tuples are output from the Join, they are printed out on the command line.

2.7. Query Parser

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:

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), 
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

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:

3. Logistics

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

3.1. Collaboration

This lab can be completed alone or with a partner. Please indicate clearly who you worked with, if anyone, on your individual writeup.

3.2. Submitting your assignment

For all deadlines besides the final version, you only need to submit cs133-lab2.tar.gz tarball (such that, untarred, it creates a cs133-lab2/src/java/simpledb directory with your code). You can use the ant handin target to generate the tarball.

For the final version of the lab, the files you need to submit are:

Submit all your files for Exercise 1 under "Lab 2: Part 1", Exercises 2-3 under "Lab 2: Part 2" and the final version of the lab under "Lab 2: Final" on Sakai.
If you are working with a partner, only one person needs to submit on Sakai. However, be sure to indicate the name of your partner in the writeup.

3.3 Grading

Your grade for the lab will be based on the final version after all exercises are complete.

75% 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 produces no errors (passes all of the tests) from both ant test and ant systemtest.

Important: before testing, we will replace your build.xml 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 cs133-lab2.tar.gz
$ cd ./cs133-lab2
[replace build.xml and test]
$ ant test
$ ant systemtest
[additional tests]

If any of these commands fail, we'll be unhappy, and, therefore, so will your grade.

An additional 25% of your grade will be based on the quality of your writeup, our subjective evaluation of your code, and on-time submission for the intermediate deadlines.



Thanks to our friends and colleagues at MIT and UWashington for doing all the heavy lifting on creating SimpleDB!