I have come across a strange behavior while executing a SELECT query using Statement#executeUpdate() by mistake. While the Javadoc clearly states that executeUpdate() throws SQLException if the given SQL statement produces a ResultSet object. But when I'm executing SELECT * from TABLE_NAME, I don't get any exception. Instead I'm getting an return value which is same as the no. of rows selected, if no. is less than or equal to 10. If the no. is more than 10, the return value is always 10.
Connection conn;
Statement stmt;
try {
conn = getConnection();
stmt = conn.createStatement();
int count = stmt.executeUpdate("SELECT * from TABLE_NAME");
log.info("row count: " + count);
} catch (SQLException e) {
log.error(e);
// handle exception
} finally {
DbUtils.closeQuietly(stmt);
DbUtils.closeQuietly(conn);
}
I am using Oracle 10g.
Am I missing something here or is it up to the drivers to define their own behavior?
This behaviour is definetely contradicts Statement.executeUpdate API. What's interesting,
java.sql.Driver.jdbcCompliant API says "A driver may only report true here if it passes the JDBC compliance tests". I tested oracle.jdbc.OracleDriver.jdbcCompliant - it returns true. I also tested com.mysql.jdbc.Driver.jdbcCompliant - it returns false. But in the same situation as you describe it throws
Exception in thread "main" java.sql.SQLException: Can not issue SELECT via executeUpdate().
It seems that JDBC drivers are unpredictable.
According to the specifications Statement.executeUpdate() method returns the row count for SQL Data Manipulation Language (DML).
UPD: I attempted to make an assumption about the returned result (which is always <=10). It seems, that the oracle statement's implementation returns here a number of a such called premature batch count (according to the decompiled sources OraclePreparedStatement class). This is somehow linked to the update statements. May be this value equals 10 by default.
UPD-2: According to this: Performance Extensions: The premature batch flush count is summed to the return value of the next executeUpdate() or sendBatch() method.
The query you are using doesn't produce a ResultSet but affects Rows obviously. That's why you don't get an SQLException but a count of the no of rows affected. The mystery is why it doesn't go beyond 10. May it is Oracle JDBC Driver Implementation Specific.
Your sql query is to retrieve all rows from table_name. So, you can use execute() method instead of executeUpdate() method. Because later method generally use when your task is related database manipulating language like update query.
use
int count = stmt.executeQuery("SELECT * from TABLE_NAME");
instead of
int count = stmt.executeUpdate("SELECT * from TABLE_NAME");
for getting total no. of rows.
Related
What is the difference between statement.executeQuery and statement.getResultSet(). I believe both will return ResultSet for a select statement but are there any specific criteria when we should use which of them.
In general you should use executeQuery if you know you are executing a select statement. The getResultSet() method by itself does not execute the statement.
The getResultSet is intended to be used in combination with execute. The execute methods are intended for use with unknown statement types, or statements that can produce multiple results (that is 0 or more update counts or result sets).
So in short: you should normally use executeQuery.
A simple example when you should use execute if the code doesn't know what query it is going to execute (an update, a query, or something more complex), for example when executing user provided queries.
Another example are SQL Server stored procedures, which can return multiple update counts and result sets.
A generic way of processing a result of execute is:
boolean isResultSet = statement.execute(sql);
while (true) {
if (isResultSet) {
try (ResultSet rs = statement.getResultSet()) {
// do something with result set
}
} else {
int updateCount = statement.getUpdateCount();
if (updateCount == -1) {
// -1 signals no more results
break;
}
// do something with update count
}
// move to next result
isResultSet = statement.getMoreResults();
}
This ensures that all* results get processed.
*: This example ignores exception results for systems (like SQL Server) that allow multiple exceptions to be interleaved with result sets and update counts, see How to get *everything* back from a stored procedure using JDBC for a more thorough example
Check out the JavaDoc for these methods. getResultSet can return null but executeQuery never return null.
There are also more constraints. For example executeQuery cannot be called on a PreparedStatement or CallableStatement.
I'm trying to do batch inserts for a couple of records using a PreparedStatement. However, I keep getting false whenever I call next on my ResultSet
public ResultSet insert_into_batch(ArrayList<Movie> values) throws SQLException
{
conn.setAutoCommit(false);
ArrayList<String> added = new ArrayList<String>();
String stmt = "INSERT INTO movies (id,title,year,director) VALUES (?,?,?,?)";
PreparedStatement psInsertRecord = conn.prepareStatement(stmt, Statement.RETURN_GENERATED_KEYS);
for (Movie movie : values)
{
if (!added.contains(movie.getId())) {
added.add(movie.getId());
psInsertRecord.setString(1, movie.getId());
psInsertRecord.setString(2, movie.getTitle());
psInsertRecord.setInt(3, movie.getYear());
psInsertRecord.setString(4, movie.getDirector());
psInsertRecord.addBatch();
}
}
psInsertRecord.executeBatch();
conn.commit();
conn.setAutoCommit(true);
return psInsertRecord.getGeneratedKeys();
}
I see three potential problems with your code.
You insert a record with an explicit id, so no keys might have been generated. Generated keys are for when the database system generates the identifier, but your code generates it explicitly (some database systems will then still generate the result set, but not all, not sure about MySQL in this case).
You commit the connection directly after batch execution, as a result the generated keys result set (if any) created by the execute will already have been closed or relevant information cleared. Although I would have expected a driver to throw an exception when calling getGeneratedKeys() in that situation.
The JDBC specification states that it is implementation-defined whether getGeneratedKeys is supported for batch execution, so it is possible that it is not supported at all (but a quick look at the sources of MySQL Connector/J suggests it is supported).
I am trying to insert into a db that I have, and I'd like to do so through parameters. I am connecting to a postgres db using java.
I can connect to the db just fine. I know that because I have various operations that I am using that are already working were I can see, and update existing rows in my db. I am having trouble with INSERT.
I have the following:
private String _update_rentals = "INSERT into rentals (cid, mid) values (?,?)";
private PreparedStatement _update_rentals_statement;
private String _update_movie_status = "UPDATE Movie SET checkedout = true WHERE mid = ?";
private PreparedStatement _update_movie_status_statement;
And I initialize them:
_update_movie_status_statement = _customer_db.prepareStatement(_update_movie_status);
_update_rentals_statement = _customer_db.prepareStatement(_update_rentals);
And
while (movieAvail.next()){
System.out.println(movieAvail.getBoolean(1));
if (movieAvail.getBoolean(1) == false){
//Do chekcout
_update_rentals_statement.clearParameters();
_update_rentals_statement.setInt(1, cid);
_update_rentals_statement.setInt(2, mid);
_update_rentals_statement.executeQuery();
_update_movie_status_statement.clearParameters();
_update_movie_status_statement.setInt(1, mid);
_update_movie_status_statement.executeQuery();
System.out.println("Enjoy your movie!");
}
}
I am getting an error with both of the executeQuery() calls. For some reason I am getting the following error with both:
Exception in thread "main" org.postgresql.util.PSQLException: No results were returned by the query.
I looked at other posts, and I believed that I was following syntax for both insert/ update correctly, so maybe I am overlooking some aspect of this.
This is all part of a larger code base, so I did not want to include the methods these pieces of code are in. But these are the isolated instances which play a part with this code.
In general, when you execute a query, you are willing to retrieve some kind of information from the database. This is usually the case when you are executing SELECT queries. However, with INSERT and UPDATE statements, you are not querying the database, you are simply executing an update or inserting new rows. In the documentation of PreparedStatement you can see in which cases an exception is being thrown when you try to call executeQuery:
Throws: SQLException - if a database access error occurs; this method
is called on a closed PreparedStatement or the SQL statement does not
return a ResultSet object
So in your case the problem is that your statements do not return a ResultSet. You should use execute or executeUpdate instead. The former simply executes the update, while the latter does the same, but also returns the number of affected rows.
I think the main issue is that you are calling executeQuery(), which expects a result to be returned, but Insert/Update are not queries and don't return a result. Try just calling execute().
I think this is a bug.
I'm using latest MySQL JDBC library.
I have multiple threads. Each thread execute a query and for each row add a batch to a prepared statement.
Sometimes the instruction "stmt.setLong(i, aLong)" launch a NullPointerException.
stmt,i and aLong are not null.
PreparedStatement stmt = db.prepareStatement("myinsert");
while (rs.next()) {
long aLong = rs.getLong(1);
...
stmt.setLong(1,aLong);
stmt.addBatch();
}
Here is the exception:
java.lang.NullPointerException
at com.mysql.jdbc.ConnectionImpl.getServerCharacterEncoding(ConnectionImpl.java:3124)
at com.mysql.jdbc.PreparedStatement.setInternal(PreparedStatement.java:3729)
at com.mysql.jdbc.PreparedStatement.setLong(PreparedStatement.java:3751)
at org.apache.commons.dbcp2.DelegatingPreparedStatement.setLong(DelegatingPreparedStatement.java:127)
at org.apache.commons.dbcp2.DelegatingPreparedStatement.setLong(DelegatingPreparedStatement.java:127)
at com.mypackage.MyClass$MyThread.run(MyClass.java:117)
If I launch only one thread, it works.
The exception also occurs without apache dbcp2 library.
I'm going crazy!
I solved the problem removing these lines of codes before creation of the ResultSet
Statement stmt = Database.getDatabase().createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(Integer.MIN_VALUE);
From MySQL documentation here.
The combination of a forward-only, read-only result set, with a fetch size of Integer.MIN_VALUE serves as a signal to the driver to stream result sets row-by-row. After this, any result sets created with the statement will be retrieved row-by-row.
I have this really big table with some millions of records every day and in the end of every day I am extracting all the records of the previous day. I am doing this like:
String SQL = "select col1, col2, coln from mytable where timecol = yesterday";
Statement.executeQuery(SQL);
The problem is that this program takes like 2GB of memory because it takes all the results in memory then it processes it.
I tried setting the Statement.setFetchSize(10) but it takes exactly the same memory from OS it does not make any difference. I am using Microsoft SQL Server 2005 JDBC Driver for this.
Is there any way to read the results in small chunks like the Oracle database driver does when the query is executed to show only a few rows and as you scroll down more results are shown?
In JDBC, the setFetchSize(int) method is very important to performance and memory-management within the JVM as it controls the number of network calls from the JVM to the database and correspondingly the amount of RAM used for ResultSet processing.
Inherently if setFetchSize(10) is being called and the driver is ignoring it, there are probably only two options:
Try a different JDBC driver that will honor the fetch-size hint.
Look at driver-specific properties on the Connection (URL and/or property map when creating the Connection instance).
The RESULT-SET is the number of rows marshalled on the DB in response to the query.
The ROW-SET is the chunk of rows that are fetched out of the RESULT-SET per call from the JVM to the DB.
The number of these calls and resulting RAM required for processing is dependent on the fetch-size setting.
So if the RESULT-SET has 100 rows and the fetch-size is 10,
there will be 10 network calls to retrieve all of the data, using roughly 10*{row-content-size} RAM at any given time.
The default fetch-size is 10, which is rather small.
In the case posted, it would appear the driver is ignoring the fetch-size setting, retrieving all data in one call (large RAM requirement, optimum minimal network calls).
What happens underneath ResultSet.next() is that it doesn't actually fetch one row at a time from the RESULT-SET. It fetches that from the (local) ROW-SET and fetches the next ROW-SET (invisibly) from the server as it becomes exhausted on the local client.
All of this depends on the driver as the setting is just a 'hint' but in practice I have found this is how it works for many drivers and databases (verified in many versions of Oracle, DB2 and MySQL).
The fetchSize parameter is a hint to the JDBC driver as to many rows to fetch in one go from the database. But the driver is free to ignore this and do what it sees fit. Some drivers, like the Oracle one, fetch rows in chunks, so you can read very large result sets without needing lots of memory. Other drivers just read in the whole result set in one go, and I'm guessing that's what your driver is doing.
You can try upgrading your driver to the SQL Server 2008 version (which might be better), or the open-source jTDS driver.
You need to ensure that auto-commit on the Connection is turned off, or setFetchSize will have no effect.
dbConnection.setAutoCommit(false);
Edit: Remembered that when I used this fix it was Postgres-specific, but hopefully it will still work for SQL Server.
Statement interface Doc
SUMMARY: void setFetchSize(int rows)
Gives the JDBC driver a hint as to the
number of rows that should be fetched
from the database when more rows are
needed.
Read this ebook J2EE and beyond By Art Taylor
Sounds like mssql jdbc is buffering the entire resultset for you. You can add a connect string parameter saying selectMode=cursor or responseBuffering=adaptive. If you are on version 2.0+ of the 2005 mssql jdbc driver then response buffering should default to adaptive.
http://msdn.microsoft.com/en-us/library/bb879937.aspx
It sounds to me that you really want to limit the rows being returned in your query and page through the results. If so, you can do something like:
select * from (select rownum myrow, a.* from TEST1 a )
where myrow between 5 and 10 ;
You just have to determine your boundaries.
Try this:
String SQL = "select col1, col2, coln from mytable where timecol = yesterday";
connection.setAutoCommit(false);
PreparedStatement stmt = connection.prepareStatement(SQL, SQLServerResultSet.TYPE_SS_SERVER_CURSOR_FORWARD_ONLY, SQLServerResultSet.CONCUR_READ_ONLY);
stmt.setFetchSize(2000);
stmt.set....
stmt.execute();
ResultSet rset = stmt.getResultSet();
while (rset.next()) {
// ......
I had the exact same problem in a project. The issue is that even though the fetch size might be small enough, the JDBCTemplate reads all the result of your query and maps it out in a huge list which might blow your memory. I ended up extending NamedParameterJdbcTemplate to create a function which returns a Stream of Object. That Stream is based on the ResultSet normally returned by JDBC but will pull data from the ResultSet only as the Stream requires it. This will work if you don't keep a reference of all the Object this Stream spits. I did inspire myself a lot on the implementation of org.springframework.jdbc.core.JdbcTemplate#execute(org.springframework.jdbc.core.ConnectionCallback). The only real difference has to do with what to do with the ResultSet. I ended up writing this function to wrap up the ResultSet:
private <T> Stream<T> wrapIntoStream(ResultSet rs, RowMapper<T> mapper) {
CustomSpliterator<T> spliterator = new CustomSpliterator<T>(rs, mapper, Long.MAX_VALUE, NON-NULL | IMMUTABLE | ORDERED);
Stream<T> stream = StreamSupport.stream(spliterator, false);
return stream;
}
private static class CustomSpliterator<T> extends Spliterators.AbstractSpliterator<T> {
// won't put code for constructor or properties here
// the idea is to pull for the ResultSet and set into the Stream
#Override
public boolean tryAdvance(Consumer<? super T> action) {
try {
// you can add some logic to close the stream/Resultset automatically
if(rs.next()) {
T mapped = mapper.mapRow(rs, rowNumber++);
action.accept(mapped);
return true;
} else {
return false;
}
} catch (SQLException) {
// do something with this Exception
}
}
}
you can add some logic to make that Stream "auto closable", otherwise don't forget to close it when you are done.