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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-3753) select from table with integer primary key and blob column does not do sort avoidance
Date Mon, 25 Oct 2010 18:10:22 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3753?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-3753:
----------------------------------


rather than sort avoidance what might be more interesting is if there is any way to avoid
sending the blob to the sorter.  I would not be surprised if the most efficient way to execute
this query is to do the sequential scan of the base table doing the length calculation and
finally only sending the (key, blob_length) through the sorter - 
since every single row of the table is looked at.  The sequential scan might be more i/o efficient
than random
probes from the index.

Defintely should test whether 1 vs. 1000 rows is any difference.

I would not be surprised if sort estimates were not including the blob, but for some reason
the sort is including the blob.  Should run it against trunk to see if it still breaks given
all the blob optimization that has been done in last few releases.

> select from table with integer primary key and blob column does not do sort avoidance
> -------------------------------------------------------------------------------------
>
>                 Key: DERBY-3753
>                 URL: https://issues.apache.org/jira/browse/DERBY-3753
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.3.3.0, 10.4.1.3, 10.5.1.1
>            Reporter: Kathey Marsden
>         Attachments: derby.log, Derby3753.java
>
>
> In working on DERBY-3732 I noticed that select from a table with a primary key and a
blob column does not do sort avoidance.  If you  remove the optimizer directive, this fixture
in BlobMemTest will run out of memory sorting if memory.BlobMemTest is run with 16M heap.
>  private void testBlobLength(boolean lengthless) throws SQLException, IOException, IllegalArgumentException,
IllegalAccessException, InvocationTargetException {
>         getConnection().setAutoCommit(false);
>         Statement s = createStatement();
>         s.executeUpdate("CREATE TABLE BLOBTAB (K INT CONSTRAINT PK PRIMARY KEY, B BLOB("
+ LONG_BLOB_LENGTH + "))");
>         
>         PreparedStatement ps = prepareStatement("INSERT INTO BLOBTAB VALUES(?,?)");
>         // We allocate 16MB for the test so use something bigger than that.
>         ps.setInt(1,1);
>         LoopingAlphabetStream stream = new LoopingAlphabetStream(LONG_BLOB_LENGTH);
>         if (lengthless) {
>             Method m = null;
>             try {
>                 Class c = ps.getClass();
>                 m = c.getMethod("setBinaryStream",new Class[] {Integer.TYPE,
>                             InputStream.class});                
>             } catch (NoSuchMethodException e) {
>                 // ignore method not found as method may not be present for 
>                 // jdk's lower than 1.6.
>                 println("Skipping lengthless insert because method is not available");
>                 return;                
>             }
>             m.invoke(ps, new Object[] {new Integer(2),stream});
>         }
>         else
>             ps.setBinaryStream(2, stream,LONG_BLOB_LENGTH);
>         ps.executeUpdate();
>         // insert a zero length blob.
>         ps.setInt(1, 2);
>         ps.setBytes(2, new byte[] {});
>         ps.executeUpdate();
>         // insert a null blob.
>         ps.setInt(1, 3);
>         ps.setBytes(2,null);
>         ps.executeUpdate();
>         // insert a short blob
>         ps.setInt(1, 4);
>         ps.setBytes(2, SHORT_BLOB_BYTES);
>         ps.executeUpdate();
>         // Currently need to use optimizer override to force use of the index.
>         // Derby should use sort avoidance and do it automatically, but there
>         // appears to be a bug.
>         ResultSet rs = s.executeQuery("SELECT K, LENGTH(B), B FROM BLOBTAB" +
>                 "-- DERBY-PROPERTIES constraint=pk\n ORDER BY K"); 
>         rs.next();
>         assertEquals(LONG_BLOB_LENGTH_STRING,rs.getString(2));
>         // make sure we can still access the blob after getting length.
>         // It should be ok because we reset the stream
>         InputStream rsstream = rs.getBinaryStream(3);
>         int len= 0;
>         byte[] buf = new byte[32672];
>         for (;;)  {
>                 int size = rsstream.read(buf);
>                 if (size == -1)
>                         break;
>                 len += size;
>                 int expectedValue = ((len -1) % 26) + 'a';
>                 if (size != 0)
>                     assertEquals(expectedValue,buf[size -1]);      
>         }
>         assertEquals(LONG_BLOB_LENGTH,len);
>         // empty blob
>         rs.next();
>         assertEquals("0",rs.getString(2));
>         byte[] bytes = rs.getBytes(3);
>         assertEquals(0, bytes.length);
>         // null blob
>         rs.next();
>         assertEquals(null,rs.getString(2));
>         bytes = rs.getBytes(3);
>         assertEquals(null,bytes);
>         // short blob
>         rs.next();
>         assertEquals("3",rs.getString(2));
>         bytes = rs.getBytes(3);
>         assertTrue(Arrays.equals(SHORT_BLOB_BYTES, bytes));
>         rs.close();         
>         
>         // Select just length without selecting the blob.
>         rs = s.executeQuery("SELECT K, LENGTH(B)  FROM BLOBTAB " +
>                 "ORDER BY K");
>         JDBC.assertFullResultSet(rs, new String [][] {{"1",LONG_BLOB_LENGTH_STRING},{"2","0"},
>                 {"3",null},{"4","3"}});
>     }

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