Statement statement = ...obtain statement from Connection...
statement.execute("set partialResultsMode true");Partial Results Mode
You can use a "partial results" query mode with the Teiid Server. In this mode, the behavior of the query processor changes so that the server returns results even when some data sources are unavailable.
For example, suppose that two data sources exist for different suppliers and your data designers have created a virtual group that creates a union between the information from the two suppliers. If your application submits a query without using partial results query mode and one of the suppliers’ databases is down, the query against the virtual group returns an exception. However, if your application runs the same query in "partial results" query mode, the server returns data from the running data source and no data from the data source that is down.
When using "partial results" mode, if a source throws an exception during processing it does not cause the user’s query to fail. Rather, that source is treated as returning no more rows after the failure point. Most commonly, that source will return 0 rows.
This behavior is most useful when using UNION or OUTER JOIN queries as these operations handle missing information in a useful way. Most other kinds of queries will simply return 0 rows to the user when used in partial results mode and the source is unavailable.
For each source that is excluded from the query, a warning will be generated describing the source and the failure. These warnings can be obtained from the Statement.getWarnings() method. This method returns a SQLWarning object but in the case of "partial results" warnings, this will be an object of type org.teiid.jdbc.PartialResultsWarning class. This class can be used to obtain a list of all the failed sources by name and to obtain the specific exception thrown by each source.
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Note
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Because Teiid enables cursoring before an entire result is formed, it is possible that a data source failure will not be determined until after the first batch of results have been returned to the client. This can happen in the case of unions, but not joins. To ensure that all warnings have been accumulated, the statement should be checked after the entire result set has been read. |
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Note
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If other warnings are returned by execution, then the partial results warnings may occur after the first warning in the warning chain. |
Partial results mode is off by default but can be turned on for all queries in a Connection with either setPartialResultsMode("true") on a DataSource or partialResultsMode=true on a JDBC URL. In either case, partial results mode may be toggled later with a SET Statement.
Setting Partial Results Mode
Getting Partial Results Warnings
statement.execute("set partialResultsMode true");
ResultSet results = statement.executeQuery("SELECT Name FROM Accounts");
while (results.next()) {
... //process the result set
}
SQLWarning warning = statement.getWarnings();
while(warning != null) {
if (warning instanceof PartialResultsWarning) {
PartialResultsWarning partialWarning = (PartialResultsWarning)warning;
Collection failedConnectors = partialWarning.getFailedConnectors();
Iterator iter = failedConnectors.iterator();
while(iter.hasNext()) {
String connectorName = (String) iter.next();
SQLException connectorException = partialWarning.getConnectorException(connectorName);
System.out.println(connectorName + ": " + connectorException.getMessage());
}
}
warning = warning.getNextWarning();
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Warning
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In some instances, typically JDBC sources, the source not being initially available will prevent Teiid from automatically determining the appropriate set of source capabilities. If you get an exception indicating that the capabilities for an unavailable source are not valid in partial results mode, then it may be necessary to manually set the database version or similar property on the translator to ensure that the capabilities are known even if the source is not available. |