org.hsqldb.jdbc
Class jdbcResultSetMetaData

java.lang.Object
  extended byorg.hsqldb.jdbc.jdbcResultSetMetaData
All Implemented Interfaces:
java.sql.ResultSetMetaData

public class jdbcResultSetMetaData
extends java.lang.Object
implements java.sql.ResultSetMetaData

An object that can be used to get information about the types and properties of the columns in a ResultSet object. The following code fragment creates the ResultSet object rs, creates the ResultSetMetaData object rsmd, and uses rsmd to find out how many columns rs has and whether the first column in rs can be used in a WHERE clause.


 ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM TABLE2");
 ResultSetMetaData rsmd = rs.getMetaData();
 int numberOfColumns = rsmd.getColumnCount();
 boolean b = rsmd.isSearchable(1);

 

HSQLDB-Specific Information:

HSQLDB supports a subset of the ResultSetMetaData interface.

The JDBC specification for ResultSetMetaData is in part very vague. This causes potential incompatibility between interpretations of the specification as realized in different JDBC driver implementations. As such, deciding to what degree reporting ResultSetMetaData is accurate has been considered very carefully. Hopefully, the design decisions made in light of these considerations have yeilded precisely the subset of full ResultSetMetaData support that is most commonly needed and that is most important, while also providing, under the most common use-cases, the fastest access with the least overhead and the best comprimise between speed, accuracy, jar-foootprint and retention of JDBC resources.

(fredt@users)
(boucherb@users)

Author:
boucherb@users
See Also:
jdbcStatement.executeQuery(java.lang.String), jdbcStatement.getResultSet(), ResultSetMetaData

Field Summary
 
Fields inherited from interface java.sql.ResultSetMetaData
columnNoNulls, columnNullable, columnNullableUnknown
 
Method Summary
 java.lang.String getCatalogName(int column)
           Gets the designated column's table's catalog name.
 java.lang.String getColumnClassName(int column)
           Returns the fully-qualified name of the Java class whose instances are manufactured if the method ResultSet.getObject is called to retrieve a value from the column.
 int getColumnCount()
           Returns the number of columns in this ResultSet object.
 int getColumnDisplaySize(int column)
           Indicates the designated column's normal maximum width in characters.
 java.lang.String getColumnLabel(int column)
           Gets the designated column's suggested title for use in printouts and displays.
 java.lang.String getColumnName(int column)
           Get the designated column's name.
 int getColumnType(int column)
           Retrieves the designated column's SQL type.
 java.lang.String getColumnTypeName(int column)
           Retrieves the designated column's database-specific type name.
 int getPrecision(int column)
           Get the designated column's number of decimal digits.
 int getScale(int column)
           Gets the designated column's number of digits to right of the decimal point.
 java.lang.String getSchemaName(int column)
           Get the designated column's table's schema.
 java.lang.String getTableName(int column)
           Gets the designated column's table name.
 boolean isAutoIncrement(int column)
           Indicates whether the designated column is automatically numbered, thus read-only.
 boolean isCaseSensitive(int column)
           Indicates whether a column's case matters.
 boolean isCurrency(int column)
           Indicates whether the designated column is a cash value.
 boolean isDefinitelyWritable(int column)
           Indicates whether a write on the designated column will definitely succeed.
 int isNullable(int column)
           Indicates the nullability of values in the designated column.
 boolean isReadOnly(int column)
           Indicates whether the designated column is definitely not writable.
 boolean isSearchable(int column)
           Indicates whether the designated column can be used in a where clause.
 boolean isSigned(int column)
           Indicates whether values in the designated column are signed numbers.
 boolean isWritable(int column)
           Indicates whether it is possible for a write on the designated column to succeed.
 java.lang.String toString()
           
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Method Detail

getColumnCount

public int getColumnCount()
                   throws java.sql.SQLException
Returns the number of columns in this ResultSet object.

Specified by:
getColumnCount in interface java.sql.ResultSetMetaData
Returns:
the number of columns
Throws:
java.sql.SQLException - if a database access error occurs

isAutoIncrement

public boolean isAutoIncrement(int column)
                        throws java.sql.SQLException
Indicates whether the designated column is automatically numbered, thus read-only.

HSQLDB-Specific Information:

HSQLDB 1.7.1 did not report this value accurately, either always throwing or always returning false, depending upon client property values.

Starting with HSQLDB 1.7.2, this feature is better supported.


However, it must be stated here that, contrary to the generic documentation above, HSQLDB automatically numbered columns (IDENTITY columns, in HSQLDB parlance) are not read-only.

In fact, the generic documentation above seems to contradict the general definition of what, at minimum, an auto-increment column is:

Simply, an auto-increment column is one that guarantees it has a unique value after a successful insert or update operation, even if no value is supplied or NULL is explicitly specified by the application or a default value expression.

Further, without SQL Feature T176, Sequence generator support, the attributes of the internal source consulted for unique values are not defined. That is, unlike for a standard SQL SEQUENCE object or a system with full SQL 9x or 200n support for SQL Feature T174, Identity columns, an application must not assume and cannot determine in a standard way that auto-increment values start at any particular point, increment by any particular value or have any of the other attributes generally associated with SQL SEQUENCE generators. Further still, without full support for both feature T174 and T176, if a unique value is supplied by an application or provided by a declared or implicit default value expression, then whether that value is used or substituted with one from the automatic unique value source is implementation-defined and cannot be known in a standard way. Finally, without full support for features T174 and T176, it is also implementation-defined and cannot be know in a standard way whether an exception is thrown or a unique value is automatically substituted when an application or default value expression supplies a non-NULL, non-unique value.

Up to and including HSQLDB 1.7.2, values supplied by an application or default value expression are used if they are indeed non-NULL unique values, while an exception is thrown if either possible value source for the site attempts to supply a non-NULL, non-unique value. This is very likely to remain the behaviour of HSQLDB for its foreseable lifetime and at the very least for the duration of the 1.7.x release series.


Regardless of the new and better support for reporting isAutoIncrement(), it is still possible under certain conditions that accurate reporting may be impossible. For example, if this object's parent Connection is closed before the first call to this method or to any other method of this class that initializes the connection-dependent ResultSetMetaData values, then it is impossible to report accurately for result set columns that directly represent table column values.

Under such special circumstances, the driver rethrows the exception that occured during the initialization, or a SQLException wrapping it.

Those wishing to determine the auto-increment status of a table column in isolation from ResultSetMetaData can do so by inspecting the corresponding value of the SYSTEM_COLUMNS.IS_IDENTITY BOOLEAN column which is also currently included (in a fashion proprietary to HSQLDB) as the last column of the jdbcDatabaseMetaData.getColumns() result.

Specified by:
isAutoIncrement in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isCaseSensitive

public boolean isCaseSensitive(int column)
                        throws java.sql.SQLException
Indicates whether a column's case matters.

HSQLDB-Specific Information:

HSQLDB 1.7.1 did not report this value accurately.

Starting with 1.7.2, this feature is better supported.

This method returns true for any column whose data type is a character type, with the exception of VARCHAR_IGNORECASE for which it returns false. It also returns false for any column whose data type is a not a character data type.

Specified by:
isCaseSensitive in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isSearchable

public boolean isSearchable(int column)
                     throws java.sql.SQLException
Indicates whether the designated column can be used in a where clause.

HSQLDB-Specific Information:

HSQLDB 1.7.1 did not report this value accurately.

Starting with 1.7.2, this feature is better supported.

If the data type of the column is definitely known to be searchable in any way under HSQLDB, then true is returned, else false. That is, if the type is reported in DatabaseMetaData.getTypeInfo() as having DatabaseMetaData.typePredNone or is not reported, then false is returned, else true.

Specified by:
isSearchable in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isCurrency

public boolean isCurrency(int column)
                   throws java.sql.SQLException
Indicates whether the designated column is a cash value.

HSQLDB-Specific Information:

Up to and including HSQLDB 1.7.2, this method always returns false.

This is because true fixed (precision,scale) data types are not yet supported. That is, DECIMAL and NUMERIC types are implemented as a thin wrap of java.math.BigDecimal, which cannot, without additional, as yet unimplemented constraint enforcement code, be said to be a fixed (precision,scale) types.

Specified by:
isCurrency in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isNullable

public int isNullable(int column)
               throws java.sql.SQLException
Indicates the nullability of values in the designated column.

HSQLDB-Specific Information:

Up to 1.7.1, HSQLDB did not report this value accurately.

Starting with 1.7.2, this feature is better supported.

columnNullableUnknown is always returned for result set columns that do not directly represent table column values (i.e. are calculated), while the corresponding value in SYSTEM_COLUMNS.NULLABLE is returned for result set columns that do directly represent table column values.

Those wishing to determine the nullable status of a table column in isolation from ResultSetMetaData and in a DBMS-independent fashion can do so by calling DatabaseMetaData.getColumns() with the appropriate filter values and inspecting the result at the position described in the API documentation.

Specified by:
isNullable in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
the nullability status of the given column; one of columnNoNulls, columnNullable or columnNullableUnknown
Throws:
java.sql.SQLException - if a database access error occurs

isSigned

public boolean isSigned(int column)
                 throws java.sql.SQLException
Indicates whether values in the designated column are signed numbers.

HSQLDB-Specific Information:

HSQLDB 1.7.1 introduced support for this feature and 1.7.2 reports identical values (although using a slightly different implementation).

Specified by:
isSigned in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

getColumnDisplaySize

public int getColumnDisplaySize(int column)
                         throws java.sql.SQLException
Indicates the designated column's normal maximum width in characters.

HSQLDB-Specific Information:

Up to and including HSQLDB 1.7.1, this method always returned 0, which was intended to convey unknown display size. Unfortunately, this value is not universally handled by all clients and in the worst case can cause some applications to crash.

Starting with 1.7.2, this feature is better supported.

The current calculation follows these rules:

  1. Long character types and datetime types:

    The maximum length/precision, repectively.

  2. CHAR and VARCHAR types:

    • If the result set column is a direct pass through of a table column value, column size was declared and the connection is to an embedded database instance, then the declared value is returned.

    • Otherwise, the value of the system property hsqldb.max_xxxchar_display_size or the magic value 32766 (0x7FFE) (tested usable/accepted by most tools and compatible with assumptions made by java.io read/write UTF) when the system property is not defined or is not accessible, due to security constraints.

    It must be noted that the latter value in no way affects the ability of the HSQLDB JDBC driver to retrieve longer values and serves only as the current best effort at providing a value that maximizes usability across a wide range of tools, given that the HSQLDB database engine does not require the length to be declared and does not necessarily enforce it, even if declared.

  3. Number types:

    The max precision, plus the length of the negation character (1), plus (if applicable) the maximum number of characters that may occupy the exponent character sequence. Note that some legacy tools do not correctly handle BIGINT values of greater than 18 digits.

  4. BOOLEAN (BIT) type:

    The length of the character sequence "false" (5), the longer of the two boolean value String representations.

  5. Remaining types:

    The maximum length/precision, respectively, as reported by DatabaseMetaData.getTypeInfo(), when applicable. If the maximum display size is unknown, unknowable or inapplicable, then zero is returned.

Specified by:
getColumnDisplaySize in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
the normal maximum number of characters allowed as the width of the designated column
Throws:
java.sql.SQLException - if a database access error occurs

getColumnLabel

public java.lang.String getColumnLabel(int column)
                                throws java.sql.SQLException
Gets the designated column's suggested title for use in printouts and displays.

HSQLDB-Specific Information:

In HSQLDB a ResultSet column label is determined in the following order of precedence:

  1. The label (alias) specified in the generating query.
  2. The name of the underlying column, if no label is specified.
    This also applies to aggregate functions.
  3. An empty String.

Specified by:
getColumnLabel in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
the suggested column title
Throws:
java.sql.SQLException - if a database access error occurs

getColumnName

public java.lang.String getColumnName(int column)
                               throws java.sql.SQLException
Get the designated column's name.

HSQLDB-Specific Information:

In HSQLDB a ResultSet column name is determined in the following order of prcedence:

  1. The name of the underlying columnm, if the ResultSet column represents a column in a table.
  2. The label or alias specified in the generating query.
  3. An empty String.

If the jdbc.get_column_name property of the database has been set to false, this method returns the same value as getColumnLabel(int).

Specified by:
getColumnName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
column name
Throws:
java.sql.SQLException - if a database access error occurs

getSchemaName

public java.lang.String getSchemaName(int column)
                               throws java.sql.SQLException
Get the designated column's table's schema.

HSQLDB-Specific Information:

Up to 1.7.1, HSQLDB did not support the notion of schemas at all, including schema names in result set metadata; this method always returned "".

Staring with 1.7.2, schema name reporting is supported only as an optional, experimental feature that is disabled by default. Enabling this feature requires setting the database property "hsqldb.schemas=true".

Specifically, when this feature is enabled under 1.7.2, only very limited support is provided by the engine for executing SQL containing schema-qualified database object identifiers. That is, when this feature is enabled under 1.7.2, it is not yet possible in most cases to use what would otherwise be the correct, canonical SQL calculated from ResultSetMetaData.

Regardless, reporting is done only in system table content and is not yet carried over to ResultSetMetaData.

For greater detail, see discussion at: jdbcDatabaseMetaData.

Specified by:
getSchemaName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
schema name or "" if not applicable
Throws:
java.sql.SQLException - if a database access error occurs

getPrecision

public int getPrecision(int column)
                 throws java.sql.SQLException
Get the designated column's number of decimal digits.

HSQLDB-Specific Information:

Up to and including HSQLDB 1.7.1, this method always returned 0, which was intended to convey that the precision was unknown, undefined or that no applicable limit was imposed.

Starting with 1.7.2, HSQLDB reports the maximum length or precision intrinsic to the data type of each result set column.

This method does not yet make any attempt to report the declared length or precision specifiers for table columns (if they are defined, which up to 1.7.2 is not a requirement in DDL), as these values may or may not be enforced, depending on the value of the database property:

 sql.enforce_size
 
Because the property may change from one instantiation of a Database to the next and because, when set true, is not applied to existing values in table columns (only to new values introduced by following inserts and updates), the length and/or precision specifiers for table columns still do not neccessarily accurately reflect true constraints upon the contents of the columns. This situation may or may not change in a future release.

Specified by:
getPrecision in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
precision
Throws:
java.sql.SQLException - if a database access error occurs

getScale

public int getScale(int column)
             throws java.sql.SQLException
Gets the designated column's number of digits to right of the decimal point.

HSQLDB-Specific Information:

Up to and including HSQLDB 1.7.2, this method always returns 0 (which is the actual scale for integral types and is to be interpreted as unknown or not applicable for all other types).

HSQLDB currently implements DECIMAL and NUMERIC--the only HSQLDB types to which this value would apply, if supported--as a thin wrap of BigDecimal and thus does not presently ever enforce scale declarations. Those wishing to determine the declared--intended, but not enforced--scale of a DECIMAL and NUMERIC table column should instead consult the DatabaseMetaData.getColumns() result using the required filter parameters.

Specified by:
getScale in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
scale
Throws:
java.sql.SQLException - if a database access error occurs

getTableName

public java.lang.String getTableName(int column)
                              throws java.sql.SQLException
Gets the designated column's table name.

Specified by:
getTableName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
table name or "" if not applicable
Throws:
java.sql.SQLException - if a database access error occurs

getCatalogName

public java.lang.String getCatalogName(int column)
                                throws java.sql.SQLException
Gets the designated column's table's catalog name.

HSQLDB-Specific Information:

Up to and including 1.7.1, HSQLDB did not support the notion of catalog and this method always returned "".

Starting with 1.7.2, HSQLDB supports catalog reporting only as an optional, experimental feature that is disabled by default. Enabling this feature requires setting the database property "hsqldb.catalogs=true". When enabled, the catalog name for table columns is reported as the name by which the hosting Database knows itself.

HSQLDB does not yet support any use of catalog qualification in DLL or DML. This fact is accurately indicated in the corresponding DatabaseMetaData.supportsXXX() method return values.

Regardless, reporting is done only in system table content and is not yet carried over to ResultSetMetaData.

For greater detail, see discussion at: jdbcDatabaseMetaData.

Specified by:
getCatalogName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
the name of the catalog for the table in which the given column appears or "" if not applicable
Throws:
java.sql.SQLException - if a database access error occurs

getColumnType

public int getColumnType(int column)
                  throws java.sql.SQLException
Retrieves the designated column's SQL type.

HSQLDB-Specific Information:

This reports the SQL type of the column. HSQLDB can return Objects in any Java integral type wider than Integer for an SQL integral type.

Specified by:
getColumnType in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
SQL type from java.sql.Types
Throws:
java.sql.SQLException - if a database access error occurs
See Also:
Types

getColumnTypeName

public java.lang.String getColumnTypeName(int column)
                                   throws java.sql.SQLException
Retrieves the designated column's database-specific type name.

Specified by:
getColumnTypeName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
type name used by the database. If the column type is a user-defined type, then a fully-qualified type name is returned.
Throws:
java.sql.SQLException - if a database access error occurs

isReadOnly

public boolean isReadOnly(int column)
                   throws java.sql.SQLException
Indicates whether the designated column is definitely not writable.

HSQLDB-Specific Information:

Up to and including 1.7.1, HSQLDB did not report this value accurately.

Starting with HSQLDB 1.7.2, this feature is better supported.

For result set columns that do not directly represent table column values (i.e. are calculated), true is reported. Otherwise, the read only status of the table and the database are used in the calculation, but not the read-only status of the session.

Specified by:
isReadOnly in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isWritable

public boolean isWritable(int column)
                   throws java.sql.SQLException
Indicates whether it is possible for a write on the designated column to succeed.

HSQLDB-Specific Information:

Up to and including 1.7.1, HSQLDB did not report this value accurately.

Starting with HSQLDB 1.7.2, this feature is better supported.

In essense, the negation of isReadOnly() is reported.

Specified by:
isWritable in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

isDefinitelyWritable

public boolean isDefinitelyWritable(int column)
                             throws java.sql.SQLException
Indicates whether a write on the designated column will definitely succeed.

HSQLDB-Specific Information:

HSQLDB 1.7.1 did not report this value accurately.

Starting with HSQLDB 1.7.2, this method always returns false. The reason for this is that it is generally either very complex or simply impossible to calculate deterministically true for table columns under all concievable conditions. The value is of dubious usefulness, except perhaps if there were support for updateable result sets using "SELECT ... FOR UPDATE" style locking. However, this is not anticipated to occur any time in the 1.7.x release series.

Specified by:
isDefinitelyWritable in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
true if so; false otherwise
Throws:
java.sql.SQLException - if a database access error occurs

getColumnClassName

public java.lang.String getColumnClassName(int column)
                                    throws java.sql.SQLException
Returns the fully-qualified name of the Java class whose instances are manufactured if the method ResultSet.getObject is called to retrieve a value from the column. ResultSet.getObject may return a subclass of the class returned by this method.

HSQLDB-Specific Information:

HSQLDB 1.7.1 did not support this feature; calling this method always caused an SQLException to be thrown, stating that the function was not supported.

Specified by:
getColumnClassName in interface java.sql.ResultSetMetaData
Parameters:
column - the first column is 1, the second is 2, ...
Returns:
the fully-qualified name of the class in the Java programming language that would be used by the method ResultSet.getObject to retrieve the value in the specified column. This is the class name used for custom mapping.
Throws:
java.sql.SQLException - if a database access error occurs
Since:
JDK 1.2 (JDK 1.1.x developers: read the new overview for jdbcResultSet)

toString

public java.lang.String toString()


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