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## org.hibernate.dialect
Class DB2Dialect

    java.lang.Object
      org.hibernate.dialect.Dialect
          org.hibernate.dialect.DB2Dialect

All Implemented Interfaces:
  ConversionContext

Direct Known Subclasses:
  DB2390Dialect, DB2400Dialect, DerbyDialect

    public class DB2Dialect

extends Dialect

An SQL dialect for DB2.

Field Summary

Fields inherited from class org.hibernate.dialect.Dialect
`CLOSED_QUOTE, DEFAULT_BATCH_SIZE, LEGACY_LOB_MERGE_STRATEGY, NEW_LOCATOR_LOB_MERGE_STRATEGY, NO_BATCH, QUOTE, STREAM_XFER_LOB_MERGE_STRATEGY`

Constructor Summary
`DB2Dialect()`

Method Summary
`int` | `convertToFirstRowValue(int zeroBasedFirstResult)` DB2 does have a one-based offset, however this was actually already handled in the limit string building (the '?+1' bit).
`boolean` | `doesReadCommittedCauseWritersToBlockReaders()` For the underlying database, is READ_COMMITTED isolation implemented by forcing readers to wait for write locks to be released?
`boolean` | `dropConstraints()` Do we need to drop constraints before dropping tables in this dialect?
`String` | `generateTemporaryTableName(String baseTableName)` Generate a temporary table name given the base table.
`String` | `getAddColumnString()` The syntax used to add a column to a table (optional).
`String` | `getCreateSequenceString(String sequenceName)` Typically dialects which support sequences can create a sequence with a single command.
`String` | `getCreateTemporaryTablePostfix()` Get any fragments needing to be postfixed to the command for temporary table creation.
`String` | `getCreateTemporaryTableString()` Command used to create a temporary table.
`String` | `getCrossJoinSeparator()`
`String` | `getCurrentTimestampSelectString()` Retrieve the command used to retrieve the current timestamp from the database.
`String` | `getDropSequenceString(String sequenceName)` Typically dialects which support sequences can drop a sequence with a single command.
`String` | `getForUpdateString()` Get the string to append to SELECT statements to acquire locks for this dialect.
`String` | `getIdentityColumnString()` The syntax used during DDL to define a column as being an IDENTITY.
`String` | `getIdentityInsertString()` The keyword used to insert a generated value into an identity column (or null).
`String` | `getIdentitySelectString()` Get the select command to use to retrieve the last generated IDENTITY value.
`String` | `getLimitString(String sql, int offset, int limit)` Given a limit and an offset, apply the limit clause to the query.
`String` | `getLowercaseFunction()` The name of the SQL function that transforms a string to lowercase
`String` | `getQuerySequencesString()` Get the select command used retrieve the names of all sequences.
`ResultSet` | `getResultSet(CallableStatement ps)` Given a callable statement previously processed by `Dialect.registerResultSetOutParameter(java.sql.CallableStatement, int)`, extract the `ResultSet` from the OUT parameter.
`String` | `getSelectClauseNullString(int sqlType)` Given a `Types` type code, determine an appropriate null value to use in a select clause.
`String` | `getSequenceNextValString(String sequenceName)` Generate the appropriate select statement to to retrieve the next value of a sequence.
`boolean` | `isCurrentTimestampSelectStringCallable()` Should the value returned by `Dialect.getCurrentTimestampSelectString()` be treated as callable.
`static void` | `main(String[] args)`
`int` | `registerResultSetOutParameter(CallableStatement statement, int col)` Registers an OUT parameter which will be returning a `ResultSet`.
`boolean` | `requiresCastingOfParametersInSelectClause()` DB2 in fact does require that parameters appearing in the select clause be wrapped in cast() calls to tell the DB parser the type of the select value.
`boolean` | `supportsCommentOn()`
`boolean` | `supportsCurrentTimestampSelection()` Does this dialect support a way to retrieve the database's current timestamp value?
`boolean` | `supportsEmptyInList()` Does this dialect support empty IN lists? For example, is [where XYZ in ()] a supported construct?
`boolean` | `supportsExistsInSelect()` Does the dialect support an exists statement in the select clause?
`boolean` | `supportsIdentityColumns()` Does this dialect support identity column key generation?
`boolean` | `supportsLimit()` Does this dialect support some form of limiting query results via a SQL clause?
`boolean` | `supportsLobValueChangePropogation()` Does the dialect support propagating changes to LOB values back to the database? Talking about mutating the internal value of the locator as opposed to supplying a new locator instance...
`boolean` | `supportsNotNullUnique()`
`boolean` | `supportsOuterJoinForUpdate()` Does this dialect support `FOR UPDATE` in conjunction with outer joined rows?
`boolean` | `supportsParametersInInsertSelect()` Does this dialect support parameters within the `SELECT` clause of `INSERT ...`
`boolean` | `supportsPooledSequences()` Does this dialect support "pooled" sequences.
`boolean` | `supportsResultSetPositionQueryMethodsOnForwardOnlyCursor()` Does this dialect support asking the result set its positioning information on forward only cursors.
`boolean` | `supportsSequences()` Does this dialect support sequences?
`boolean` | `supportsTemporaryTables()` Does this dialect support temporary tables?
`boolean` | `supportsTupleDistinctCounts()` Does this dialect support `count(distinct a,b)`?
`boolean` | `supportsUnionAll()` Does this dialect support UNION ALL, which is generally a faster variant of UNION?
`boolean` | `supportsVariableLimit()` Does this dialect support bind variables (i.e., prepared statement parameters) for its limit/offset?
`boolean` | `useMaxForLimit()` Does the `LIMIT` clause take a "maximum" row number instead of a total number of returned rows? This is easiest understood via an example.

Methods inherited from class org.hibernate.dialect.Dialect
`appendIdentitySelectToInsert, appendLockHint, applyLocksToSql, areStringComparisonsCaseInsensitive, bindLimitParametersFirst, bindLimitParametersInReverseOrder, buildSQLExceptionConversionDelegate, buildSQLExceptionConverter, closeQuote, createCaseFragment, createOuterJoinFragment, doesRepeatableReadCauseReadersToBlockWriters, dropTemporaryTableAfterUse, forceLimitUsage, forUpdateOfColumns, getAddForeignKeyConstraintString, getAddPrimaryKeyConstraintString, getAddUniqueConstraintString, getCascadeConstraintsString, getCaseInsensitiveLike, getCastTypeName, getColumnAliasExtractor, getColumnComment, getCreateMultisetTableString, getCreateSequenceString, getCreateSequenceStrings, getCreateSequenceStrings, getCreateTableString, getCurrentTimestampSQLFunctionName, getDefaultProperties, getDialect, getDialect, getDropForeignKeyString, getDropSequenceStrings, getDropTemporaryTableString, getForUpdateNowaitString, getForUpdateNowaitString, getForUpdateString, getForUpdateString, getForUpdateString, getForUpdateString, getFunctions, getHibernateTypeName, getHibernateTypeName, getIdentityColumnString, getIdentitySelectString, getKeywords, getLimitString, getLobMergeStrategy, getLockingStrategy, getMaxAliasLength, getNativeIdentifierGeneratorClass, getNoColumnsInsertString, getNullColumnString, getReadLockString, getSelectGUIDString, getSelectSequenceNextValString, getSqlTypeDescriptorOverride, getTableComment, getTableTypeString, getTypeName, getTypeName, getViolatedConstraintNameExtracter, getWriteLockString, hasAlterTable, hasDataTypeInIdentityColumn, hasSelfReferentialForeignKeyBug, isLockTimeoutParameterized, openQuote, performTemporaryTableDDLInIsolation, qualifyIndexName, quote, registerColumnType, registerColumnType, registerFunction, registerHibernateType, registerHibernateType, registerKeyword, remapSqlTypeDescriptor, replaceResultVariableInOrderByClauseWithPosition, supportsBindAsCallableArgument, supportsCascadeDelete, supportsCaseInsensitiveLike, supportsCircularCascadeDeleteConstraints, supportsColumnCheck, supportsExpectedLobUsagePattern, supportsIfExistsAfterTableName, supportsIfExistsBeforeTableName, supportsInsertSelectIdentity, supportsLimitOffset, supportsLockTimeouts, supportsRowValueConstructorSyntax, supportsRowValueConstructorSyntaxInInList, supportsSubqueryOnMutatingTable, supportsSubselectAsInPredicateLHS, supportsTableCheck, supportsTupleCounts, supportsUnboundedLobLocatorMaterialization, supportsUnique, supportsUniqueConstraintInCreateAlterTable, toBooleanValueString, toString, transformSelectString, useInputStreamToInsertBlob`

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

Constructor Detail

### DB2Dialect

    public DB2Dialect()

Method Detail

### getLowercaseFunction

    public String getLowercaseFunction()

  Description copied from class: `Dialect`
  The name of the SQL function that transforms a string to lowercase

  Overrides:
    `getLowercaseFunction` in class `Dialect`
  Returns:
    The dialect-specific lowercase function.

### getAddColumnString

    public String getAddColumnString()

  Description copied from class: `Dialect`
  The syntax used to add a column to a table (optional).

  Overrides:
    `getAddColumnString` in class `Dialect`
  Returns:
    The "add column" fragment.

### dropConstraints

    public boolean dropConstraints()

  Description copied from class: `Dialect`
  Do we need to drop constraints before dropping tables in this dialect?

  Overrides:
    `dropConstraints` in class `Dialect`
  Returns:
    True if constraints must be dropped prior to dropping the table; false otherwise.

### supportsIdentityColumns

    public boolean supportsIdentityColumns()

  Description copied from class: `Dialect`
  Does this dialect support identity column key generation?

  Overrides:
    `supportsIdentityColumns` in class `Dialect`
  Returns:
    True if IDENTITY columns are supported; false otherwise.

### getIdentitySelectString

    public String getIdentitySelectString()

  Description copied from class: `Dialect`
  Get the select command to use to retrieve the last generated IDENTITY value.

  Overrides:
    `getIdentitySelectString` in class `Dialect`
  Returns:
    The appropriate select command

### getIdentityColumnString

    public String getIdentityColumnString()

  Description copied from class: `Dialect`
  The syntax used during DDL to define a column as being an IDENTITY.

  Overrides:
    `getIdentityColumnString` in class `Dialect`
  Returns:
    The appropriate DDL fragment.

### getIdentityInsertString

    public String getIdentityInsertString()

  Description copied from class: `Dialect`
  The keyword used to insert a generated value into an identity column (or null). Need if the dialect does not support inserts that specify no column values.

  Overrides:
    `getIdentityInsertString` in class `Dialect`
  Returns:
    The appropriate keyword.

### getSequenceNextValString

    public String getSequenceNextValString(String sequenceName)

  Description copied from class: `Dialect`
  Generate the appropriate select statement to to retrieve the next value of a sequence.

  This should be a "stand alone" select statement.

  Overrides:
    `getSequenceNextValString` in class `Dialect`
  Parameters:
    `sequenceName` - the name of the sequence
  Returns:
    String The "nextval" select string.

### getCreateSequenceString

    public String getCreateSequenceString(String sequenceName)

  Description copied from class: `Dialect`
  Typically dialects which support sequences can create a sequence with a single command. This is convenience form of `Dialect.getCreateSequenceStrings(java.lang.String)` to help facilitate that.

  Dialects which support sequences and can create a sequence in a single command need *only* override this method. Dialects which support sequences but require multiple commands to create a sequence should instead override `Dialect.getCreateSequenceStrings(java.lang.String)`.

  Overrides:
    `getCreateSequenceString` in class `Dialect`
  Parameters:
    `sequenceName` - The name of the sequence
  Returns:
    The sequence creation command

### getDropSequenceString

    public String getDropSequenceString(String sequenceName)

  Description copied from class: `Dialect`
  Typically dialects which support sequences can drop a sequence with a single command. This is convenience form of `Dialect.getDropSequenceStrings(java.lang.String)` to help facilitate that.

  Dialects which support sequences and can drop a sequence in a single command need *only* override this method. Dialects which support sequences but require multiple commands to drop a sequence should instead override `Dialect.getDropSequenceStrings(java.lang.String)`.

  Overrides:
    `getDropSequenceString` in class `Dialect`
  Parameters:
    `sequenceName` - The name of the sequence
  Returns:
    The sequence drop commands

### supportsSequences

    public boolean supportsSequences()

  Description copied from class: `Dialect`
  Does this dialect support sequences?

  Overrides:
    `supportsSequences` in class `Dialect`
  Returns:
    True if sequences supported; false otherwise.

### supportsPooledSequences

    public boolean supportsPooledSequences()

  Description copied from class: `Dialect`
  Does this dialect support "pooled" sequences. Not aware of a better name for this. Essentially can we specify the initial and increment values?

  Overrides:
    `supportsPooledSequences` in class `Dialect`
  Returns:
    True if such "pooled" sequences are supported; false otherwise.
  See Also:
    `Dialect.getCreateSequenceStrings(String, int, int)`, `Dialect.getCreateSequenceString(String, int, int)`

### getQuerySequencesString

    public String getQuerySequencesString()

  Description copied from class: `Dialect`
  Get the select command used retrieve the names of all sequences.

  Overrides:
    `getQuerySequencesString` in class `Dialect`
  Returns:
    The select command; or null if sequences are not supported.
  See Also:
    `SchemaUpdate`

### supportsLimit

    public boolean supportsLimit()

  Description copied from class: `Dialect`
  Does this dialect support some form of limiting query results via a SQL clause?

  Overrides:
    `supportsLimit` in class `Dialect`
  Returns:
    True if this dialect supports some form of LIMIT.

### supportsVariableLimit

    public boolean supportsVariableLimit()

  Description copied from class: `Dialect`
  Does this dialect support bind variables (i.e., prepared statement parameters) for its limit/offset?

  Overrides:
    `supportsVariableLimit` in class `Dialect`
  Returns:
    True if bind variables can be used; false otherwise.

### getLimitString

    public String getLimitString(String sql,
                                 int offset,
                                 int limit)

  Description copied from class: `Dialect`
  Given a limit and an offset, apply the limit clause to the query.

  Overrides:
    `getLimitString` in class `Dialect`
  Parameters:
    `sql` - The query to which to apply the limit.
    `offset` - The offset of the limit
    `limit` - The limit of the limit ;)
  Returns:
    The modified query statement with the limit applied.

### convertToFirstRowValue

    public int convertToFirstRowValue(int zeroBasedFirstResult)

  DB2 does have a one-based offset, however this was actually already handled in the limit string building (the '?+1' bit). To not mess up inheritors, I'll leave that part alone and not touch the offset here.

  Overrides:
    `convertToFirstRowValue` in class `Dialect`
  Parameters:
    `zeroBasedFirstResult` - The user-supplied, zero-based offset
  Returns:
    zeroBasedFirstResult
  See Also:
    `Query.setFirstResult(int)`, `Criteria.setFirstResult(int)`

### getForUpdateString

    public String getForUpdateString()

  Description copied from class: `Dialect`
  Get the string to append to SELECT statements to acquire locks for this dialect.

  Overrides:
    `getForUpdateString` in class `Dialect`
  Returns:
    The appropriate `FOR UPDATE` clause string.

### useMaxForLimit

    public boolean useMaxForLimit()

  Description copied from class: `Dialect`
  Does the `LIMIT` clause take a "maximum" row number instead of a total number of returned rows?

  This is easiest understood via an example. Consider you have a table with 20 rows, but you only want to retrieve rows number 11 through 20. Generally, a limit with offset would say that the offset = 11 and the limit = 10 (we only want 10 rows at a time); this is specifying the total number of returned rows. Some dialects require that we instead specify offset = 11 and limit = 20, where 20 is the "last" row we want relative to offset (i.e. total number of rows = 20 - 11 = 9)

  So essentially, is limit relative from offset? Or is limit absolute?

  Overrides:
    `useMaxForLimit` in class `Dialect`
  Returns:
    True if limit is relative from offset; false otherwise.

### supportsOuterJoinForUpdate

    public boolean supportsOuterJoinForUpdate()

  Description copied from class: `Dialect`
  Does this dialect support `FOR UPDATE` in conjunction with outer joined rows?

  Overrides:
    `supportsOuterJoinForUpdate` in class `Dialect`
  Returns:
    True if outer joined rows can be locked via `FOR UPDATE`.

### supportsNotNullUnique

    public boolean supportsNotNullUnique()

  Overrides:
    `supportsNotNullUnique` in class `Dialect`

### supportsExistsInSelect

    public boolean supportsExistsInSelect()

  Description copied from class: `Dialect`
  Does the dialect support an exists statement in the select clause?

  Overrides:
    `supportsExistsInSelect` in class `Dialect`
  Returns:
    True if exists checks are allowed in the select clause; false otherwise.

### getSelectClauseNullString

    public String getSelectClauseNullString(int sqlType)

  Description copied from class: `Dialect`
  Given a `Types` type code, determine an appropriate null value to use in a select clause.

  One thing to consider here is that certain databases might require proper casting for the nulls here since the select here will be part of a UNION/UNION ALL.

  Overrides:
    `getSelectClauseNullString` in class `Dialect`
  Parameters:
    `sqlType` - The `Types` type code.
  Returns:
    The appropriate select clause value fragment.

### main

    public static void main(String[] args)

### supportsUnionAll

    public boolean supportsUnionAll()

  Description copied from class: `Dialect`
  Does this dialect support UNION ALL, which is generally a faster variant of UNION?

  Overrides:
    `supportsUnionAll` in class `Dialect`
  Returns:
    True if UNION ALL is supported; false otherwise.

### registerResultSetOutParameter

    public int registerResultSetOutParameter(CallableStatement statement,
                                             int col)
                                      throws SQLException

  Description copied from class: `Dialect`
  Registers an OUT parameter which will be returning a `ResultSet`. How this is accomplished varies greatly from DB to DB, hence its inclusion (along with `Dialect.getResultSet(java.sql.CallableStatement)`) here.

  Overrides:
    `registerResultSetOutParameter` in class `Dialect`
  Parameters:
    `statement` - The callable statement.
    `col` - The bind position at which to register the OUT param.
  Returns:
    The number of (contiguous) bind positions used.
  Throws:
    `SQLException` - Indicates problems registering the OUT param.

### getResultSet

    public ResultSet getResultSet(CallableStatement ps)
                           throws SQLException

  Description copied from class: `Dialect`
  Given a callable statement previously processed by `Dialect.registerResultSetOutParameter(java.sql.CallableStatement, int)`, extract the `ResultSet` from the OUT parameter.

  Overrides:
    `getResultSet` in class `Dialect`
  Parameters:
    `ps` - The callable statement.
  Returns:
    The extracted result set.
  Throws:
    `SQLException` - Indicates problems extracting the result set.

### supportsCommentOn

    public boolean supportsCommentOn()

  Overrides:
    `supportsCommentOn` in class `Dialect`

### supportsTemporaryTables

    public boolean supportsTemporaryTables()

  Description copied from class: `Dialect`
  Does this dialect support temporary tables?

  Overrides:
    `supportsTemporaryTables` in class `Dialect`
  Returns:
    True if temp tables are supported; false otherwise.

### getCreateTemporaryTableString

    public String getCreateTemporaryTableString()

  Description copied from class: `Dialect`
  Command used to create a temporary table.

  Overrides:
    `getCreateTemporaryTableString` in class `Dialect`
  Returns:
    The command used to create a temporary table.

### getCreateTemporaryTablePostfix

    public String getCreateTemporaryTablePostfix()

  Description copied from class: `Dialect`
  Get any fragments needing to be postfixed to the command for temporary table creation.

  Overrides:
    `getCreateTemporaryTablePostfix` in class `Dialect`
  Returns:
    Any required postfix.

### generateTemporaryTableName

    public String generateTemporaryTableName(String baseTableName)

  Description copied from class: `Dialect`
  Generate a temporary table name given the base table.

  Overrides:
    `generateTemporaryTableName` in class `Dialect`
  Parameters:
    `baseTableName` - The table name from which to base the temp table name.
  Returns:
    The generated temp table name.

### supportsCurrentTimestampSelection

    public boolean supportsCurrentTimestampSelection()

  Description copied from class: `Dialect`
  Does this dialect support a way to retrieve the database's current timestamp value?

  Overrides:
    `supportsCurrentTimestampSelection` in class `Dialect`
  Returns:
    True if the current timestamp can be retrieved; false otherwise.

### getCurrentTimestampSelectString

    public String getCurrentTimestampSelectString()

  Description copied from class: `Dialect`
  Retrieve the command used to retrieve the current timestamp from the database.

  Overrides:
    `getCurrentTimestampSelectString` in class `Dialect`
  Returns:
    The command.

### isCurrentTimestampSelectStringCallable

    public boolean isCurrentTimestampSelectStringCallable()

  Description copied from class: `Dialect`
  Should the value returned by `Dialect.getCurrentTimestampSelectString()` be treated as callable. Typically this indicates that JDBC escape syntax is being used...

  Overrides:
    `isCurrentTimestampSelectStringCallable` in class `Dialect`
  Returns:
    True if the `Dialect.getCurrentTimestampSelectString()` return is callable; false otherwise.

### supportsParametersInInsertSelect

    public boolean supportsParametersInInsertSelect()

  Does this dialect support parameters within the `SELECT` clause of `INSERT ... SELECT ...` statements?

  DB2 is know to support parameters in the `SELECT` clause, but only in casted form (see `requiresCastingOfParametersInSelectClause()`).

  Overrides:
    `supportsParametersInInsertSelect` in class `Dialect`
  Returns:
    True.

### requiresCastingOfParametersInSelectClause

    public boolean requiresCastingOfParametersInSelectClause()

  DB2 in fact does require that parameters appearing in the select clause be wrapped in cast() calls to tell the DB parser the type of the select value.

  Overrides:
    `requiresCastingOfParametersInSelectClause` in class `Dialect`
  Returns:
    True.

### supportsResultSetPositionQueryMethodsOnForwardOnlyCursor

    public boolean supportsResultSetPositionQueryMethodsOnForwardOnlyCursor()

  Description copied from class: `Dialect`
  Does this dialect support asking the result set its positioning information on forward only cursors. Specifically, in the case of scrolling fetches, Hibernate needs to use `ResultSet.isAfterLast()` and `ResultSet.isBeforeFirst()`. Certain drivers do not allow access to these methods for forward only cursors.

  NOTE : this is highly driver dependent!

  Overrides:
    `supportsResultSetPositionQueryMethodsOnForwardOnlyCursor` in class `Dialect`
  Returns:
    True if methods like `ResultSet.isAfterLast()` and `ResultSet.isBeforeFirst()` are supported for forward only cursors; false otherwise.

### getCrossJoinSeparator

    public String getCrossJoinSeparator()

  Overrides:
    `getCrossJoinSeparator` in class `Dialect`
  Returns:
    Returns the separator to use for defining cross joins when translating HQL queries.

    Typically this will be either [`cross join` ] or [`,` ]

    Note that the spaces are important!

### supportsEmptyInList

    public boolean supportsEmptyInList()

  Description copied from class: `Dialect`
  Does this dialect support empty IN lists?

  For example, is [where XYZ in ()] a supported construct?

  Overrides:
    `supportsEmptyInList` in class `Dialect`
  Returns:
    True if empty in lists are supported; false otherwise.

### supportsLobValueChangePropogation

    public boolean supportsLobValueChangePropogation()

  Description copied from class: `Dialect`
  Does the dialect support propagating changes to LOB values back to the database? Talking about mutating the internal value of the locator as opposed to supplying a new locator instance...

  For BLOBs, the internal value might be changed by: `Blob.setBinaryStream(long)`, `Blob.setBytes(long, byte[])`, `Blob.setBytes(long, byte[], int, int)`, or `Blob.truncate(long)`.

  For CLOBs, the internal value might be changed by: `Clob.setAsciiStream(long)`, `Clob.setCharacterStream(long)`, `Clob.setString(long, String)`, `Clob.setString(long, String, int, int)`, or `Clob.truncate(long)`.

  NOTE : I do not know the correct answer currently for databases which (1) are not part of the cruise control process or (2) do not `Dialect.supportsExpectedLobUsagePattern()`.

  Overrides:
    `supportsLobValueChangePropogation` in class `Dialect`
  Returns:
    True if the changes are propagated back to the database; false otherwise.

### doesReadCommittedCauseWritersToBlockReaders

    public boolean doesReadCommittedCauseWritersToBlockReaders()

  Description copied from class: `Dialect`
  For the underlying database, is READ_COMMITTED isolation implemented by forcing readers to wait for write locks to be released?

  Overrides:
    `doesReadCommittedCauseWritersToBlockReaders` in class `Dialect`
  Returns:
    True if writers block readers to achieve READ_COMMITTED; false otherwise.

### supportsTupleDistinctCounts

    public boolean supportsTupleDistinctCounts()

  Description copied from class: `Dialect`
  Does this dialect support `count(distinct a,b)`?

  Overrides:
    `supportsTupleDistinctCounts` in class `Dialect`
  Returns:
    True if the database supports counting distinct tuples; false otherwise.

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