Performance

# Performance Performance Comparison PolePosition Benchmark Database Performance Tuning Using the Built-In Profiler Application Profiling Database Profiling Statement Execution Plans How Data is Stored and How Indexes Work Fast Database Import ## Performance Comparison In many cases H2 is faster than other (open source and not open source) database engines. Please note this is mostly a single connection benchmark run on one computer, with many very simple operations running against the database. This benchmark does not include very complex queries. The embedded mode of H2 is faster than the client-server mode because the per-statement overhead is greatly reduced. ### Embedded
Test Case | Unit | H2 | HSQLDB | Derby
--- | --- | --- | --- | ---
Simple: Init | ms | 1019 | 1907 | 8280
Simple: Query (random) | ms | 1304 | 873 | 1912
Simple: Query (sequential) | ms | 835 | 1839 | 5415
Simple: Update (sequential) | ms | 961 | 2333 | 21759
Simple: Delete (sequential) | ms | 950 | 1922 | 32016
Simple: Memory Usage | MB | 21 | 10 | 8
BenchA: Init | ms | 919 | 2133 | 7528
BenchA: Transactions | ms | 1219 | 2297 | 8541
BenchA: Memory Usage | MB | 12 | 15 | 7
BenchB: Init | ms | 905 | 1993 | 8049
BenchB: Transactions | ms | 1091 | 583 | 1165
BenchB: Memory Usage | MB | 17 | 11 | 8
BenchC: Init | ms | 2491 | 4003 | 8064
BenchC: Transactions | ms | 1979 | 803 | 2840
BenchC: Memory Usage | MB | 19 | 22 | 9
Executed statements | # | 1930995 | 1930995 | 1930995
Total time | ms | 13673 | 20686 | 105569
Statements per second | # | 141226 | 93347 | 18291
### Client-Server
Test Case | Unit | H2 (Server) | HSQLDB | Derby | PostgreSQL | MySQL
--- | --- | --- | --- | --- | --- | ---
Simple: Init | ms | 16338 | 17198 | 27860 | 30156 | 29409
Simple: Query (random) | ms | 3399 | 2582 | 6190 | 3315 | 3342
Simple: Query (sequential) | ms | 21841 | 18699 | 42347 | 30774 | 32611
Simple: Update (sequential) | ms | 6913 | 7745 | 28576 | 32698 | 11350
Simple: Delete (sequential) | ms | 8051 | 9751 | 42202 | 44480 | 16555
Simple: Memory Usage | MB | 22 | 11 | 9 | 0 | 1
BenchA: Init | ms | 12996 | 14720 | 24722 | 26375 | 26060
BenchA: Transactions | ms | 10134 | 10250 | 18452 | 21453 | 15877
BenchA: Memory Usage | MB | 13 | 15 | 9 | 0 | 1
BenchB: Init | ms | 15264 | 16889 | 28546 | 31610 | 29747
BenchB: Transactions | ms | 3017 | 3376 | 1842 | 2771 | 1433
BenchB: Memory Usage | MB | 17 | 12 | 11 | 1 | 1
BenchC: Init | ms | 14020 | 10407 | 17655 | 19520 | 17532
BenchC: Transactions | ms | 5076 | 3160 | 6411 | 6063 | 4530
BenchC: Memory Usage | MB | 19 | 21 | 11 | 1 | 1
Executed statements | # | 1930995 | 1930995 | 1930995 | 1930995 | 1930995
Total time | ms | 117049 | 114777 | 244803 | 249215 | 188446
Statements per second | # | 16497 | 16823 | 7887 | 7748 | 10246
### Benchmark Results and Comments #### H2 Version 1.4.177 (2014-04-12) was used for the test. For most operations, the performance of H2 is about the same as for HSQLDB. One situation where H2 is slow is large result sets, because they are buffered to disk if more than a certain number of records are returned. The advantage of buffering is: there is no limit on the result set size. #### HSQLDB Version 2.3.2 was used for the test. Cached tables are used in this test (`hsqldb.default_table_type=cached`), and the write delay is 1 second (`SET WRITE_DELAY 1`). #### Derby Version 10.10.1.1 was used for the test. Derby is clearly the slowest embedded database in this test. This seems to be a structural problem, because all operations are really slow. It will be hard for the developers of Derby to improve the performance to a reasonable level. A few problems have been identified: leaving autocommit on is a problem for Derby. If it is switched off during the whole test, the results are about 20% better for Derby. Derby calls `FileChannel.force(false)`, but only twice per log file (not on each commit). Disabling this call improves performance for Derby by about 2%. Unlike H2, Derby does not call `FileDescriptor.sync()` on each checkpoint. Derby supports a testing mode (system property `derby.system.durability=test`) where durability is disabled. According to the documentation, this setting should be used for testing only, as the database may not recover after a crash. Enabling this setting improves performance by a factor of 2.6 (embedded mode) or 1.4 (server mode). Even if enabled, Derby is still less than half as fast as H2 in default mode. #### PostgreSQL Version 9.1.5 was used for the test. The following options where changed in `postgresql.conf: fsync = off, commit_delay = 1000`. PostgreSQL is run in server mode. The memory usage number is incorrect, because only the memory usage of the JDBC driver is measured. #### MySQL Version 5.1.65-log was used for the test. MySQL was run with the InnoDB backend. The setting `innodb_flush_log_at_trx_commit` (found in the `my.ini / my.cnf` file) was set to 0. Otherwise (and by default), MySQL is slow (around 140 statements per second in this test) because it tries to flush the data to disk for each commit. For small transactions (when autocommit is on) this is really slow. But many use cases use small or relatively small transactions. Too bad this setting is not listed in the configuration wizard, and it always overwritten when using the wizard. You need to change this setting manually in the file `my.ini / my.cnf`, and then restart the service. The memory usage number is incorrect, because only the memory usage of the JDBC driver is measured. #### Firebird Firebird 1.5 (default installation) was tested, but the results are not published currently. It is possible to run the performance test with the Firebird database, and any information on how to configure Firebird for higher performance are welcome. #### Why Oracle / MS SQL Server / DB2 are Not Listed The license of these databases does not allow to publish benchmark results. This doesn't mean that they are fast. They are in fact quite slow, and need a lot of memory. But you will need to test this yourself. SQLite was not tested because the JDBC driver doesn't support transactions. ### About this Benchmark #### How to Run This test was as follows:
    build benchmark
#### Separate Process per Database For each database, a new process is started, to ensure the previous test does not impact the current test. #### Number of Connections This is mostly a single-connection benchmark. BenchB uses multiple connections; the other tests use one connection. #### Real-World Tests Good benchmarks emulate real-world use cases. This benchmark includes 4 test cases: BenchSimple uses one table and many small updates / deletes. BenchA is similar to the TPC-A test, but single connection / single threaded (see also: www.tpc.org). BenchB is similar to the TPC-B test, using multiple connections (one thread per connection). BenchC is similar to the TPC-C test, but single connection / single threaded. #### Comparing Embedded with Server Databases This is mainly a benchmark for embedded databases (where the application runs in the same virtual machine as the database engine). However MySQL and PostgreSQL are not Java databases and cannot be embedded into a Java application. For the Java databases, both embedded and server modes are tested. #### Test Platform This test is run on Mac OS X 10.6. No virus scanner was used, and disk indexing was disabled. The JVM used is Sun JDK 1.6. #### Multiple Runs When a Java benchmark is run first, the code is not fully compiled and therefore runs slower than when running multiple times. A benchmark should always run the same test multiple times and ignore the first run(s). This benchmark runs three times, but only the last run is measured. #### Memory Usage It is not enough to measure the time taken, the memory usage is important as well. Performance can be improved by using a bigger cache, but the amount of memory is limited. HSQLDB tables are kept fully in memory by default; this benchmark uses 'disk based' tables for all databases. Unfortunately, it is not so easy to calculate the memory usage of PostgreSQL and MySQL, because they run in a different process than the test. This benchmark currently does not print memory usage of those databases. #### Delayed Operations Some databases delay some operations (for example flushing the buffers) until after the benchmark is run. This benchmark waits between each database tested, and each database runs in a different process (sequentially). #### Transaction Commit / Durability Durability means transaction committed to the database will not be lost. Some databases (for example MySQL) try to enforce this by default by calling `fsync()` to flush the buffers, but most hard drives don't actually flush all data. Calling the method slows down transaction commit a lot, but doesn't always make data durable. When comparing the results, it is important to think about the effect. Many database suggest to 'batch' operations when possible. This benchmark switches off autocommit when loading the data, and calls commit after each 1000 inserts. However many applications need 'short' transactions at runtime (a commit after each update). This benchmark commits after each update / delete in the simple benchmark, and after each business transaction in the other benchmarks. For databases that support delayed commits, a delay of one second is used. #### Using Prepared Statements Wherever possible, the test cases use prepared statements. #### Currently Not Tested: Startup Time The startup time of a database engine is important as well for embedded use. This time is not measured currently. Also, not tested is the time used to create a database and open an existing database. Here, one (wrapper) connection is opened at the start, and for each step a new connection is opened and then closed. ## PolePosition Benchmark The PolePosition is an open source benchmark. The algorithms are all quite simple. It was developed / sponsored by db4o. This test was not run for a longer time, so please be aware that the results below are for older database versions (H2 version 1.1, HSQLDB 1.8, Java 1.4).
Test Case | Unit | H2 | HSQLDB | MySQL
--- | --- | --- | --- | ---
Melbourne write | ms | 369 | 249 | 2022
Melbourne read | ms | 47 | 49 | 93
Melbourne read_hot | ms | 24 | 43 | 95
Melbourne delete | ms | 147 | 133 | 176
Sepang write | ms | 965 | 1201 | 3213
Sepang read | ms | 765 | 948 | 3455
Sepang read_hot | ms | 789 | 859 | 3563
Sepang delete | ms | 1384 | 1596 | 6214
Bahrain write | ms | 1186 | 1387 | 6904
Bahrain query_indexed_string | ms | 336 | 170 | 693
Bahrain query_string | ms | 18064 | 39703 | 41243
Bahrain query_indexed_int | ms | 104 | 134 | 678
Bahrain update | ms | 191 | 87 | 159
Bahrain delete | ms | 1215 | 729 | 6812
Imola retrieve | ms | 198 | 194 | 4036
Barcelona write | ms | 413 | 832 | 3191
Barcelona read | ms | 119 | 160 | 1177
Barcelona query | ms | 20 | 5169 | 101
Barcelona delete | ms | 388 | 319 | 3287
Total | ms | 26724 | 53962 | 87112
There are a few problems with the PolePosition test: HSQLDB uses in-memory tables by default while H2 uses persistent tables. The HSQLDB version included in PolePosition does not support changing this, so you need to replace `poleposition-0.20/lib/hsqldb.jar` with a newer version (for example `hsqldb-1.8.0.7.jar`), and then use the setting `hsqldb.connecturl=jdbc:hsqldb:file:data/hsqldb/dbbench2;hsqldb.default_table_type=cached;sql.enforce_size=true` in the file `Jdbc.properties`. HSQLDB keeps the database open between tests, while H2 closes the database (losing all the cache). To change that, use the database URL `jdbc:h2:file:data/h2/dbbench;DB_CLOSE_DELAY=-1` The amount of cache memory is quite important, specially for the PolePosition test. Unfortunately, the PolePosition test does not take this into account. ## Database Performance Tuning ### Keep Connections Open or Use a Connection Pool If your application opens and closes connections a lot (for example, for each request), you should consider using a connection pool. Opening a connection using `DriverManager.getConnection` is specially slow if the database is closed. By default the database is closed if the last connection is closed. If you open and close connections a lot but don't want to use a connection pool, consider keeping a 'sentinel' connection open for as long as the application runs, or use delayed database closing. See also Closing a database. ### Use a Modern JVM Newer JVMs are faster. Upgrading to the latest version of your JVM can provide a "free" boost to performance. Switching from the default Client JVM to the Server JVM using the `-server` command-line option improves performance at the cost of a slight increase in start-up time. ### Virus Scanners Some virus scanners scan files every time they are accessed. It is very important for performance that database files are not scanned for viruses. The database engine never interprets the data stored in the files as programs, that means even if somebody would store a virus in a database file, this would be harmless (when the virus does not run, it cannot spread). Some virus scanners allow to exclude files by suffix. Ensure files ending with `.db` are not scanned. ### Using the Trace Options If the performance hot spots are in the database engine, in many cases the performance can be optimized by creating additional indexes, or changing the schema. Sometimes the application does not directly generate the SQL statements, for example if an O/R mapping tool is used. To view the SQL statements and JDBC API calls, you can use the trace options. For more information, see Using the Trace Options. ### Index Usage This database uses indexes to improve the performance of `SELECT, UPDATE, DELETE`. If a column is used in the `WHERE` clause of a query, and if an index exists on this column, then the index can be used. Multi-column indexes are used if all or the first columns of the index are used. Both equality lookup and range scans are supported. Indexes are used to order result sets, but only if the condition uses the same index or no index at all. The results are sorted in memory if required. Indexes are created automatically for primary key and unique constraints. Indexes are also created for foreign key constraints, if required. For other columns, indexes need to be created manually using the `CREATE INDEX` statement. ### How Data is Stored Internally For persistent databases, if a table is created with a single column primary key of type `BIGINT, INT, SMALLINT, TINYINT`, then the data of the table is organized in this way. This is sometimes also called a "clustered index" or "index organized table". H2 internally stores table data and indexes in the form of b-trees. Each b-tree stores entries as a list of unique keys (one or more columns) and data (zero or more columns). The table data is always organized in the form of a "data b-tree" with a single column key of type `long`. If a single column primary key of type `BIGINT, INT, SMALLINT, TINYINT` is specified when creating the table (or just after creating the table, but before inserting any rows), then this column is used as the key of the data b-tree. If no primary key has been specified, if the primary key column is of another data type, or if the primary key contains more than one column, then a hidden auto-increment column of type `BIGINT` is added to the table, which is used as the key for the data b-tree. All other columns of the table are stored within the data area of this data b-tree (except for large `BLOB, CLOB` columns, which are stored externally). For each additional index, one new "index b-tree" is created. The key of this b-tree consists of the indexed columns, plus the key of the data b-tree. If a primary key is created after the table has been created, or if the primary key contains multiple column, or if the primary key is not of the data types listed above, then the primary key is stored in a new index b-tree. ### Optimizer This database uses a cost based optimizer. For simple and queries and queries with medium complexity (less than 7 tables in the join), the expected cost (running time) of all possible plans is calculated, and the plan with the lowest cost is used. For more complex queries, the algorithm first tries all possible combinations for the first few tables, and the remaining tables added using a greedy algorithm (this works well for most joins). Afterwards a genetic algorithm is used to test at most 2000 distinct plans. Only left-deep plans are evaluated. ### Expression Optimization After the statement is parsed, all expressions are simplified automatically if possible. Operations are evaluated only once if all parameters are constant. Functions are also optimized, but only if the function is constant (always returns the same result for the same parameter values). If the `WHERE` clause is always false, then the table is not accessed at all. ### COUNT(*) Optimization If the query only counts all rows of a table, then the data is not accessed. However, this is only possible if no `WHERE` clause is used, that means it only works for queries of the form `SELECT COUNT(*) FROM table`. ### Updating Optimizer Statistics / Column Selectivity When executing a query, at most one index per join can be used. If the same table is joined multiple times, for each join only one index is used (the same index could be used for both joins, or each join could use a different index). Example: for the query `SELECT * FROM TEST T1, TEST T2 WHERE T1.NAME='A' AND T2.ID=T1.ID`, two index can be used, in this case the index on NAME for T1 and the index on ID for T2. If a table has multiple indexes, sometimes more than one index could be used. Example: if there is a table `TEST(ID, NAME, FIRSTNAME)` and an index on each column, then two indexes could be used for the query `SELECT * FROM TEST WHERE NAME='A' AND FIRSTNAME='B'`, the index on NAME or the index on FIRSTNAME. It is not possible to use both indexes at the same time. Which index is used depends on the selectivity of the column. The selectivity describes the 'uniqueness' of values in a column. A selectivity of 100 means each value appears only once, and a selectivity of 1 means the same value appears in many or most rows. For the query above, the index on NAME should be used if the table contains more distinct names than first names. The SQL statement `ANALYZE` can be used to automatically estimate the selectivity of the columns in the tables. This command should be run from time to time to improve the query plans generated by the optimizer. ### In-Memory (Hash) Indexes Using in-memory indexes, specially in-memory hash indexes, can speed up queries and data manipulation. In-memory indexes are automatically used for in-memory databases, but can also be created for persistent databases using `CREATE MEMORY TABLE`. In many cases, the rows itself will also be kept in-memory. Please note this may cause memory problems for large tables. In-memory hash indexes are backed by a hash table and are usually faster than regular indexes. However, hash indexes only supports direct lookup (`WHERE ID = ?`) but not range scan (`WHERE ID < ?`). To use hash indexes, use HASH as in: `CREATE UNIQUE HASH INDEX` and `CREATE TABLE ...(ID INT PRIMARY KEY HASH,...)`. ### Use Prepared Statements If possible, use prepared statements with parameters. ### Prepared Statements and IN(...) Avoid generating SQL statements with a variable size IN(...) list. Instead, use a prepared statement with arrays as in the following example:
    PreparedStatement prep = conn.prepareStatement(
        "SELECT * FROM TABLE(X INT=?) T INNER JOIN TEST ON T.X=TEST.ID");
    prep.setObject(1, new Object[] { "1", "2" });
    ResultSet rs = prep.executeQuery();
### Optimization Examples See `src/test/org/h2/samples/optimizations.sql` for a few examples of queries that benefit from special optimizations built into the database. ### Cache Size and Type By default the cache size of H2 is quite small. Consider using a larger cache size, or enable the second level soft reference cache. See also Cache Settings. ### Data Types Each data type has different storage and performance characteristics: The `DECIMAL/NUMERIC` type is slower and requires more storage than the `REAL` and `DOUBLE` types. Text types are slower to read, write, and compare than numeric types and generally require more storage. See Large Objects for information on `BINARY` vs. `BLOB` and `VARCHAR` vs. `CLOB` performance. Parsing and formatting takes longer for the `TIME`, `DATE`, and `TIMESTAMP` types than the numeric types. `SMALLINT/TINYINT/BOOLEAN` are not significantly smaller or faster to work with than `INTEGER` in most modes. ### Sorted Insert Optimization To reduce disk space usage and speed up table creation, an optimization for sorted inserts is available. When used, b-tree pages are split at the insertion point. To use this optimization, add `SORTED` before the `SELECT` statement:
    CREATE TABLE TEST(ID INT PRIMARY KEY, NAME VARCHAR) AS
        SORTED SELECT X, SPACE(100) FROM SYSTEM_RANGE(1, 100);
    INSERT INTO TEST
        SORTED SELECT X, SPACE(100) FROM SYSTEM_RANGE(101, 200);
## Using the Built-In Profiler A very simple Java profiler is built-in. To use it, use the following template:
    import org.h2.util.Profiler;
    Profiler prof = new Profiler();
    prof.startCollecting();
    // .... some long running process, at least a few seconds
    prof.stopCollecting();
    System.out.println(prof.getTop(3));
## Application Profiling ### Analyze First Before trying to optimize performance, it is important to understand where the problem is (what part of the application is slow). Blind optimization or optimization based on guesses should be avoided, because usually it is not an efficient strategy. There are various ways to analyze an application. Sometimes two implementations can be compared using `System.currentTimeMillis()`. But this does not work for complex applications with many modules, and for memory problems. A simple way to profile an application is to use the built-in profiling tool of java. Example:
    java -Xrunhprof:cpu=samples,depth=16 com.acme.Test
Unfortunately, it is only possible to profile the application from start to end. Another solution is to create a number of full thread dumps. To do that, first run `jps -l` to get the process id, and then run `jstack <pid>` or `kill -QUIT <pid>` (Linux) or press Ctrl+C (Windows). A simple profiling tool is included in H2. To use it, the application needs to be changed slightly. Example:
    import org.h2.util;
    ...
    Profiler profiler = new Profiler();
    profiler.startCollecting();
    // application code
    System.out.println(profiler.getTop(3));
The profiler is built into the H2 Console tool, to analyze databases that open slowly. To use it, run the H2 Console, and then click on 'Test Connection'. Afterwards, click on "Test successful" and you get the most common stack traces, which helps to find out why it took so long to connect. You will only get the stack traces if opening the database took more than a few seconds. ## Database Profiling The `ConvertTraceFile` tool generates SQL statement statistics at the end of the SQL script file. The format used is similar to the profiling data generated when using `java -Xrunhprof`. For this to work, the trace level needs to be 2 or higher (`TRACE_LEVEL_FILE=2`). The easiest way to set the trace level is to append the setting to the database URL, for example: `jdbc:h2:~/test;TRACE_LEVEL_FILE=2` or `jdbc:h2:tcp://localhost/~/test;TRACE_LEVEL_FILE=2`. As an example, execute the the following script using the H2 Console:
    SET TRACE_LEVEL_FILE 2;
    DROP TABLE IF EXISTS TEST;
    CREATE TABLE TEST(ID INT PRIMARY KEY, NAME VARCHAR(255));
    @LOOP 1000 INSERT INTO TEST VALUES(?, ?);
    SET TRACE_LEVEL_FILE 0;
After running the test case, convert the `.trace.db` file using the `ConvertTraceFile` tool. The trace file is located in the same directory as the database file.
    java -cp h2*.jar org.h2.tools.ConvertTraceFile
        -traceFile "~/test.trace.db" -script "~/test.sql"
The generated file `test.sql` will contain the SQL statements as well as the following profiling data (results vary):
    -----------------------------------------
    -- SQL Statement Statistics
    -- time: total time in milliseconds (accumulated)
    -- count: how many times the statement ran
    -- result: total update count or row count
    -----------------------------------------
    -- self accu    time   count  result sql
    --  62%  62%     158    1000    1000 INSERT INTO TEST VALUES(?, ?);
    --  37% 100%      93       1       0 CREATE TABLE TEST(ID INT PRIMARY KEY...
    --   0% 100%       0       1       0 DROP TABLE IF EXISTS TEST;
    --   0% 100%       0       1       0 SET TRACE_LEVEL_FILE 3;
## Statement Execution Plans The SQL statement `EXPLAIN` displays the indexes and optimizations the database uses for a statement. The following statements support `EXPLAIN`: `SELECT, UPDATE, DELETE, MERGE, INSERT`. The following query shows that the database uses the primary key index to search for rows:
    EXPLAIN SELECT * FROM TEST WHERE ID=1;
    SELECT
        TEST.ID,
        TEST.NAME
    FROM PUBLIC.TEST
        /* PUBLIC.PRIMARY_KEY_2: ID = 1 */
    WHERE ID = 1
For joins, the tables in the execution plan are sorted in the order they are processed. The following query shows the database first processes the table `INVOICE` (using the primary key). For each row, it will additionally check that the value of the column `AMOUNT` is larger than zero, and for those rows the database will search in the table `CUSTOMER` (using the primary key). The query plan contains some redundancy so it is a valid statement.
    CREATE TABLE CUSTOMER(ID IDENTITY, NAME VARCHAR);
    CREATE TABLE INVOICE(ID IDENTITY,
        CUSTOMER_ID INT REFERENCES CUSTOMER(ID),
        AMOUNT NUMBER);

    EXPLAIN SELECT I.ID, C.NAME FROM CUSTOMER C, INVOICE I
    WHERE I.ID=10 AND AMOUNT>0 AND C.ID=I.CUSTOMER_ID;

    SELECT
        I.ID,
        C.NAME
    FROM PUBLIC.INVOICE I
        /* PUBLIC.PRIMARY_KEY_9: ID = 10 */
        /* WHERE (I.ID = 10)
            AND (AMOUNT > 0)
        */
    INNER JOIN PUBLIC.CUSTOMER C
        /* PUBLIC.PRIMARY_KEY_5: ID = I.CUSTOMER_ID */
        ON 1=1
    WHERE (C.ID = I.CUSTOMER_ID)
        AND ((I.ID = 10)
        AND (AMOUNT > 0))
### Displaying the Scan Count `EXPLAIN ANALYZE` additionally shows the scanned rows per table and pages read from disk per table or index. This will actually execute the query, unlike `EXPLAIN` which only prepares it. The following query scanned 1000 rows, and to do that had to read 85 pages from the data area of the table. Running the query twice will not list the pages read from disk, because they are now in the cache. The `tableScan` means this query doesn't use an index.
    EXPLAIN ANALYZE SELECT * FROM TEST;
    SELECT
        TEST.ID,
        TEST.NAME
    FROM PUBLIC.TEST
        /* PUBLIC.TEST.tableScan */
        /* scanCount: 1000 */
    /*
    total: 85
    TEST.TEST_DATA read: 85 (100%)
    */
The cache will prevent the pages are read twice. H2 reads all columns of the row unless only the columns in the index are read. Except for large CLOB and BLOB, which are not store in the table. ### Special Optimizations For certain queries, the database doesn't need to read all rows, or doesn't need to sort the result even if `ORDER BY` is used. For queries of the form `SELECT COUNT(*), MIN(ID), MAX(ID) FROM TEST`, the query plan includes the line `/* direct lookup */` if the data can be read from an index. For queries of the form `SELECT DISTINCT CUSTOMER_ID FROM INVOICE`, the query plan includes the line `/* distinct */` if there is an non-unique or multi-column index on this column, and if this column has a low selectivity. For queries of the form `SELECT * FROM TEST ORDER BY ID`, the query plan includes the line `/* index sorted */` to indicate there is no separate sorting required. For queries of the form `SELECT * FROM TEST GROUP BY ID ORDER BY ID`, the query plan includes the line `/* group sorted */` to indicate there is no separate sorting required. ## How Data is Stored and How Indexes Work Internally, each row in a table is identified by a unique number, the row id. The rows of a table are stored with the row id as the key. The row id is a number of type long. If a table has a single column primary key of type `INT` or `BIGINT`, then the value of this column is the row id, otherwise the database generates the row id automatically. There is a (non-standard) way to access the row id: using the `_ROWID_` pseudo-column:
    CREATE TABLE ADDRESS(FIRST_NAME VARCHAR,
        NAME VARCHAR, CITY VARCHAR, PHONE VARCHAR);
    INSERT INTO ADDRESS VALUES('John', 'Miller', 'Berne', '123 456 789');
    INSERT INTO ADDRESS VALUES('Philip', 'Jones', 'Berne', '123 012 345');
    SELECT _ROWID_, * FROM ADDRESS;
The data is stored in the database as follows:
_ROWID_ | FIRST_NAME | NAME | CITY | PHONE
--- | --- | --- | --- | ---
1 | John | Miller | Berne | 123 456 789
2 | Philip | Jones | Berne | 123 012 345
Access by row id is fast because the data is sorted by this key. Please note the row id is not available until after the row was added (that means, it can not be used in computed columns or constraints). If the query condition does not contain the row id (and if no other index can be used), then all rows of the table are scanned. A table scan iterates over all rows in the table, in the order of the row id. To find out what strategy the database uses to retrieve the data, use `EXPLAIN SELECT`:
    SELECT * FROM ADDRESS WHERE NAME = 'Miller';

    EXPLAIN SELECT PHONE FROM ADDRESS WHERE NAME = 'Miller';
    SELECT
        PHONE
    FROM PUBLIC.ADDRESS
        /* PUBLIC.ADDRESS.tableScan */
    WHERE NAME = 'Miller';
### Indexes An index internally is basically just a table that contains the indexed column(s), plus the row id:
    CREATE INDEX INDEX_PLACE ON ADDRESS(CITY, NAME, FIRST_NAME);
In the index, the data is sorted by the indexed columns. So this index contains the following data:
CITY | NAME | FIRST_NAME | _ROWID_
--- | --- | --- | ---
Berne | Jones | Philip | 2
Berne | Miller | John | 1
When the database uses an index to query the data, it searches the index for the given data, and (if required) reads the remaining columns in the main data table (retrieved using the row id). An index on city, name, and first name (multi-column index) allows to quickly search for rows when the city, name, and first name are known. If only the city and name, or only the city is known, then this index is also used (so creating an additional index on just the city is not needed). This index is also used when reading all rows, sorted by the indexed columns. However, if only the first name is known, then this index is not used:
    EXPLAIN SELECT PHONE FROM ADDRESS
        WHERE CITY = 'Berne' AND NAME = 'Miller'
        AND FIRST_NAME = 'John';
    SELECT
        PHONE
    FROM PUBLIC.ADDRESS
        /* PUBLIC.INDEX_PLACE: FIRST_NAME = 'John'
            AND CITY = 'Berne'
            AND NAME = 'Miller'
         */
    WHERE (FIRST_NAME = 'John')
        AND ((CITY = 'Berne')
        AND (NAME = 'Miller'));

    EXPLAIN SELECT PHONE FROM ADDRESS WHERE CITY = 'Berne';
    SELECT
        PHONE
    FROM PUBLIC.ADDRESS
        /* PUBLIC.INDEX_PLACE: CITY = 'Berne' */
    WHERE CITY = 'Berne';

    EXPLAIN SELECT * FROM ADDRESS ORDER BY CITY, NAME, FIRST_NAME;
    SELECT
        ADDRESS.FIRST_NAME,
        ADDRESS.NAME,
        ADDRESS.CITY,
        ADDRESS.PHONE
    FROM PUBLIC.ADDRESS
        /* PUBLIC.INDEX_PLACE */
    ORDER BY 3, 2, 1
    /* index sorted */;

    EXPLAIN SELECT PHONE FROM ADDRESS WHERE FIRST_NAME = 'John';
    SELECT
        PHONE
    FROM PUBLIC.ADDRESS
        /* PUBLIC.ADDRESS.tableScan */
    WHERE FIRST_NAME = 'John';
If your application often queries the table for a phone number, then it makes sense to create an additional index on it:
    CREATE INDEX IDX_PHONE ON ADDRESS(PHONE);
This index contains the phone number, and the row id:
PHONE | _ROWID_
--- | ---
123 012 345 | 2
123 456 789 | 1
### Using Multiple Indexes Within a query, only one index per logical table is used. Using the condition `PHONE = '123 567 789' OR CITY = 'Berne'` would use a table scan instead of first using the index on the phone number and then the index on the city. It makes sense to write two queries and combine then using `UNION`. In this case, each individual query uses a different index:
    EXPLAIN SELECT NAME FROM ADDRESS WHERE PHONE = '123 567 789'
    UNION SELECT NAME FROM ADDRESS WHERE CITY = 'Berne';

    (SELECT
        NAME
    FROM PUBLIC.ADDRESS
        /* PUBLIC.IDX_PHONE: PHONE = '123 567 789' */
    WHERE PHONE = '123 567 789')
    UNION
    (SELECT
        NAME
    FROM PUBLIC.ADDRESS
        /* PUBLIC.INDEX_PLACE: CITY = 'Berne' */
    WHERE CITY = 'Berne')
## Fast Database Import To speed up large imports, consider using the following options temporarily: `SET LOG 0` (disabling the transaction log) `SET CACHE_SIZE` (a large cache is faster) `SET LOCK_MODE 0` (disable locking) `SET UNDO_LOG 0` (disable the session undo log) These options can be set in the database URL: `jdbc:h2:~/test;LOG=0;CACHE_SIZE=65536;LOCK_MODE=0;UNDO_LOG=0`. Most of those options are not recommended for regular use, that means you need to reset them after use. If you have to import a lot of rows, use a PreparedStatement or use CSV import. Please note that `CREATE TABLE(...) ... AS SELECT ...` is faster than `CREATE TABLE(...); INSERT INTO ... SELECT ...`.