Class IndexStatisticsDaemonImpl

java.lang.Object
org.apache.derby.impl.services.daemon.IndexStatisticsDaemonImpl
All Implemented Interfaces:
Runnable, IndexStatisticsDaemon

public class IndexStatisticsDaemonImpl extends Object implements IndexStatisticsDaemon, Runnable
Daemon acting as a coordinator for creating and updating index cardinality statistics.

The need for updated statistics is currently determined when compiling a SELECT query. The unit of work is then scheduled with this daemon, and the work itself will be carried out in a separate thread. If the worker thread doesn't exist it is created, if it is idle the unit of work will be processed immediately, and if it is busy the unit of work has to wait in the queue.

The daemon code has a notion of a background task. If the update is run as a background task, it will try to affect other activity in the Derby database as little as possible. As far as possible, it will not set locks on the conglomerates it scans, and if it needs to take locks it will give up immediately if the locks cannot be obtained. In some cases it will also roll back to release locks already taken, ad then retry. Since we are accessing shared structures the background work may still interfere with the user activity in the database due to locking, but all such operations carried out by the daemon are of short duration.

The high level flow of an update to index statistics is:

  1. schedule update (the only action carried out by the user thread)
  2. for each index:
    1. scan index
    2. invalidate statements dependent on current statistics
    3. drop existing statistics
    4. add new statistics

List of possible improvements:

  1. Reduce potential impact of multiple invalidations (per table), probably by finding a way to invalidate only once after all indexes for a table have had their statistics updated. So far invalidation has proven to be the most difficult piece of the puzzle due to the interaction with the data dictionary and sensitivity to concurrent activity for the table.

Implementation notes: List of potential cleanups before going into a release:

  1. Consider removing all tracing code. May involve improving logging if parts of the trace output is valuable enough.
  • Field Details

    • AS_BACKGROUND_TASK

      private static final boolean AS_BACKGROUND_TASK
      See Also:
    • AS_EXPLICIT_TASK

      private static final boolean AS_EXPLICIT_TASK
      See Also:
    • MAX_QUEUE_LENGTH

      private static final int MAX_QUEUE_LENGTH
      Maximum number of work units allowed in the queue.
    • logStream

      private final HeaderPrintWriter logStream
    • doLog

      private final boolean doLog
      Tells if logging is enabled.
    • doTrace

      private final boolean doTrace
      Tells if tracing is enabled.
    • traceToDerbyLog

      private final boolean traceToDerbyLog
      Tells if traces are written to the Derby log file.
    • traceToStdOut

      private final boolean traceToStdOut
      Tells if traces are written to standard out.
    • daemonDisabled

      private boolean daemonDisabled
      Tells if the daemon has been disabled.
    • ctxMgr

      private final ContextManager ctxMgr
      The context manager for the worker thread.
    • skipDisposableStats

      public final boolean skipDisposableStats
      Tells if disposable stats should be generated, which will happen in soft-upgrade mode or when the user asks us to revert to the old behavior.

      Made public to allow access for CreateIndexConstantAction and FromBaseTable, but this is no longer necessary when the debug property to keep disposable statistics is removed.

    • daemonLCC

      private LanguageConnectionContext daemonLCC
      The language connection context for the worker thread.
    • db

      private final Database db
      The database object for the database we are handling automatic index statistics update for.
    • dbOwner

      private final String dbOwner
      The name of the database owner.
    • databaseName

      private final String databaseName
    • queue

      private final ArrayList<TableDescriptor> queue
      A list of tables that shall have their index statistics updated. Note that the descriptor isn't removed before the work has been completed.
    • runningThread

      private Thread runningThread
      The thread in which the index statistics refresh operation is being executed, if any. Created as needed, but there will only be one thread doing the work. The thread is allowed to die since it is assumed that index statistics regeneration is rather infrequent.
    • errorsConsecutive

      private int errorsConsecutive
      Number of consecutive errors, used as a metric to decide if the damoen should be automatically shut down.
    • errorsUnknown

      private long errorsUnknown
    • errorsKnown

      private long errorsKnown
    • wuProcessed

      private long wuProcessed
    • wuScheduled

      private long wuScheduled
    • wuRejectedDup

      private long wuRejectedDup
    • wuRejectedFQ

      private long wuRejectedFQ
    • wuRejectedOther

      private long wuRejectedOther
    • timeOfCreation

      private final long timeOfCreation
      Specifies when the daemon was created.
    • runTime

      private long runTime
      The period of time (ms) for which the daemon has been doing active work.
    • tsb

      private final StringBuffer tsb
  • Constructor Details

    • IndexStatisticsDaemonImpl

      public IndexStatisticsDaemonImpl(HeaderPrintWriter log, boolean doLog, String traceLevel, Database db, String userName, String databaseName)
      Creates a new daemon.
      Parameters:
      log - the log to write to
      doLog - whether to log activity information
      traceLevel - whether, and to where, trace information should be written ("off|log|stdout|both")
      db - the database ("off|log|stdout|both")
      userName - the name of the database owner
      databaseName - the name of the database (not stored in the db obj)
  • Method Details

    • dbAtLeast10_9

      private boolean dbAtLeast10_9(Database db)
      Tells if the database is 10.9 or newer.
    • schedule

      public void schedule(TableDescriptor td)
      Schedules an update of the index statistics for the specified table.

      Assume the descriptor will be valid until we get around to generate the statistics. If it turns out to be invalid, it will be discarded.

      Specified by:
      schedule in interface IndexStatisticsDaemon
      Parameters:
      td - base table descriptor to update index statistics for
    • acceptWork

      private boolean acceptWork(TableDescriptor td)
      Determines if the given work can be accepted.
      Parameters:
      td - the table descriptor to check
      Returns:
      true if work can be accepted, false if not.
    • generateStatistics

      private void generateStatistics(LanguageConnectionContext lcc, TableDescriptor td) throws StandardException
      Generates index statistics for all indexes associated with the given table descriptor.

      This method is run as a background task.

      Parameters:
      lcc - connection context to use to perform the work
      td - target base table descriptor
      Throws:
      StandardException - if accessing the conglomerates fail
    • isShuttingDown

      private boolean isShuttingDown()
      Return true if we are being shutdown
    • updateIndexStatsMinion

      private void updateIndexStatsMinion(LanguageConnectionContext lcc, TableDescriptor td, ConglomerateDescriptor[] cds, boolean asBackgroundTask) throws StandardException
      Updates the index statistics for the given table and the specified indexes.

      API note: Using null to update the statistics for all conglomerates is preferred over explicitly passing an array with all the conglomerates for the table. Doing so allows for some optimizations, and will cause a disposable statistics check to be performed.

      Parameters:
      lcc - language connection context used to perform the work
      td - the table to update index stats for
      cds - the conglomerates to update statistics for (non-index conglomerates will be ignored), null means all indexes
      asBackgroundTask - whether the updates are done automatically as part of a background task or if explicitly invoked by the user
      Throws:
      StandardException - if something goes wrong
    • writeUpdatedStats

      private void writeUpdatedStats(LanguageConnectionContext lcc, TableDescriptor td, UUID index, long numRows, long[] cardinality, boolean asBackgroundTask) throws StandardException
      Writes updated statistics for the specified index to the data dictionary.
      Parameters:
      lcc - connection context to use to perform the work
      td - the base table
      index - the index of the base table
      numRows - number of rows in the base table
      cardinality - the number of unique values in the index (per number of leading columns)
      asBackgroundTask - whether the update is done automatically as part of a background task or if explicitly invoked by the user
      Throws:
      StandardException - if updating the data dictionary fails
    • invalidateStatements

      private void invalidateStatements(LanguageConnectionContext lcc, TableDescriptor td, boolean asBackgroundTask) throws StandardException
      Performs an invalidation action for the given table (the event being statistics update).
      Parameters:
      lcc - connection context to use to perform the work
      td - the table to invalidate for
      asBackgroundTask - whether the update is done automatically as part of a background task or if explicitly invoked by the user
      Throws:
      StandardException - if the invalidation request fails
    • setHeapRowEstimate

      private void setHeapRowEstimate(TransactionController tc, long tableId, long rowEstimate) throws StandardException
      Sets the row estimate for the heap conglomerate.
      Parameters:
      tc - transaction to use
      tableId - the heap table
      rowEstimate - estimate of number of rows in the table
      Throws:
      StandardException - if accessing the table fails
    • run

      public void run()
      Drives the statistics generation.

      This method will be run in a separate thread, and it will keep working as long as there is work to do. When the queue is exhausted, the method will exit (the thread dies).

      Specified by:
      run in interface Runnable
    • processingLoop

      private void processingLoop()
      Main processing loop which will compute statistics until the queue of scheduled work units has been drained.
    • runExplicitly

      public void runExplicitly(LanguageConnectionContext lcc, TableDescriptor td, ConglomerateDescriptor[] cds, String runContext) throws StandardException
      Runs the statistics update sequence explicitly as requested by the user.
      Specified by:
      runExplicitly in interface IndexStatisticsDaemon
      Parameters:
      lcc - connection context to use to perform the work
      td - the base table
      cds - the indexes to update (non-index conglomerates are ignored)
      runContext - the context in which the operation is run (i.e. 'ALTER TABLE', may be null)
      Throws:
      StandardException - if updating the index statistics fails
    • stop

      public void stop()
      Stops the daemon.

      Will also clear the queue and print runtime statistics to the log the first time the method is invoked.

      Specified by:
      stop in interface IndexStatisticsDaemon
    • handleFatalErrors

      private boolean handleFatalErrors(ContextManager cm, StandardException se)
      Handles fatal errors that will cause the daemon to be shut down.
      Parameters:
      cm - context manager
      se - the exception to handle
      Returns:
      true if the error was handled, false otherwise
    • handleExpectedErrors

      private boolean handleExpectedErrors(TableDescriptor td, StandardException se)
      Handles expected errors.

      The logging of expected errors is for observability purposes only. The daemon is capable of dealing with these errors, and no interaction from the user is expected.

      Parameters:
      se - the exception to handle
      Returns:
      true if the error was handled, false otherwise
    • handleUnexpectedErrors

      private boolean handleUnexpectedErrors(TableDescriptor td, StandardException se)
      Handles unexpected errors.

      Unexpected errors are error conditions the daemon isn't set up to handle specifically. For this reason the stack trace will be logged to allow for later investigation.

      In general it is expected that the daemon will be able to recover by dropping the current unit of work and move on to the next one (if any).

      Parameters:
      se - the exception to handle
      Returns:
      true if the error was handled, false otherwise
    • sleep

      private static void sleep(long ms)
      Puts the current thread to sleep for maximum ms milliseconds.

      No guarantee is provided for the minimum amount of time slept. If interrupted, the interrupt flag will be set again.

      Parameters:
      ms - target sleep time
    • fmtScanTimes

      private static String fmtScanTimes(long[][] timings)
      Format array of scan durations as a string.
    • log

      private void log(boolean asBackgroundTask, TableDescriptor td, String msg)
      See Also:
    • log

      private void log(boolean asBackgroundTask, TableDescriptor td, Throwable t, String msg)
      Logs the information given.

      Note that if asBackgroundTask is false, nothing will be logged currently.

      Parameters:
      asBackgroundTask - true if logging for the background daemon automatically updating stats, false if not
      td - current table descriptor being worked on, may be null
      t - raised error, may be null
      msg - the message to log
    • logAlways

      private void logAlways(TableDescriptor td, Throwable t, String msg)
      Logs the information given.
      Parameters:
      td - current table descriptor being worked on, may be null
      t - raised error, may be null
      msg - the message to log
    • trace

      private void trace(int indentLevel, String msg)
    • appendRunStats

      private void appendRunStats(StringBuffer sb)
      Appends runtime statistics to the given string buffer.
      Parameters:
      sb - the string buffer to append to
    • cardToStr

      private static String cardToStr(long[] cardinality)
      Produces a textual representation of the cardinality numbers.
      Parameters:
      cardinality - index cardinality
      Returns:
      A string.
    • extractIstatInfo

      private static String extractIstatInfo(Throwable t)
      Purely for debugging, to avoid printing too much info.
    • getContextService

      private static ContextService getContextService()
      Privileged lookup of the ContextService. Must be private so that user code can't call this entry point.