[mysql-dde] Documentation of handler changes (fwd)

  • From: Lenz Grimmer <lenz@xxxxxxxxx>
  • To: mysql-dde@xxxxxxxxxxxxx
  • Date: Thu, 15 Jun 2006 14:56:05 +0200 (CEST)

Hash: SHA1

Hi Guys,

Monty just recently pushed several changes to the MySQL 5.1 handler 
interface - this might be relevant for your work...

- -- 
 Lenz Grimmer <lenz@xxxxxxxxx>
 Community Relations Manager, EMEA
 MySQL GmbH, http://www.mysql.de/, Hamburg, Germany
 Visit the MySQL Forge at http://forge.mysql.com/

- ---------- Forwarded message ----------
Date: Wed, 7 Jun 2006 02:08:53 +0200
From: Michael Widenius <monty@xxxxxxxxx>
To:  <dev-private@xxxxxxxxx>
Subject: Documentation of handler changes


The following only have to be read by people interested in the handler
interface.  All changes are pushed in the 5.1 tree and will be
availabe as soon we have a working 5.1 tree again (should be tomorrow
evening, EET time)

- -------------

We have now done some critical cleanups in the 5.1 handler interface
to fix some of the most critical problems that has escalated during
5.1 development. The idea is to get fix some burning issues to allow
us to have a reasonable stable interface until at least 5.2 or 5.3.

All handlers included in the MySQL 5.1 source have been updated
to work (but not necessary to take advantage of all benefits of the
new interfaces).

We plan to establish some some communication channels so that all
storage engine developers can participate and influence future handler
interface changes to make this process smoother and less unexpected in
the future. More about this in the near future

The changes that has been done lately has largely been done in 4
different set of changsets. (This document intend to explain all of

- - Purge handler extension, to allow MySQL to deal with any 'not
  anymore needed' log files that the handler generates. (Automatic
  removing, archiving etc). Done by Oleksandr Byelkin.
- - New easier to use handlerton interface (to get rid of compiler
  warnings every time the handlerton interface changes). Done by
  Sergei Golubchik.
- - New auto_increment interface, to allow the handler in more detail
  inform MySQL how the auto_increments are generated (for statement
  based logging).  With this the handler can generate several
  ranges of auto_increment keys for a statement, instead of one
  sequence that was the case before, and still be able to use
  statement based replication. The new auto_increment interface
  will also provide hints to the engine of how many auto_increment
  values it expect has to be generated. Done by Guilhem Bichot.
- - Some overall handler cleanups to remove things that should have
  been fixed a long time ago. Done by Michael Widenius.

The following document will describe all of the above changes in more detail

1) Purge handler (Note: You can ignore this if your handler doesn't
   need extern log rotation)

- - With the purge handler, we have introduced a 'create_iterator'
  handlerton call that can be used to create different kind of
  iterators. For now, we have only created on iterator of type
  HA_TRANSACTLOG_ITERATOR, that can be used to get information of
  all logs used by the handler.
- - The function create_iterator should fill in slots in an iterator
  object that has function pointers to 'next' and 'destroy' methods.
  The 'next method' needs to fill in data in a struct, that is
  depending on the iterator type.
- - Example of iterator usage, can be found in
  handler.cc::example_of_iterator_using_for_logs_cleanup() and
- - We also have a new function, handler.cc::signal_log_not_needed()
  that a handler should call if wants to inform MySQL that a log
  file is not in use anymore (so that MySQL can decide if it should
  delete or archive it).

2) handlerton interface:

- - st_mysql_plugin does not store a pointer to handlerton directly,
  but to a st_mysql_storage_engine structure, which in turn has a
  pointer to the handlerton.
- - handlerton does not longer have MYSQL_HANDLERTON_INTERFACE_VERSION,
  name, comment, and init() fields. The version is in
  st_mysql_storage_engine, the other three - only in st_mysql_plugin.
- - handlerton now _can_ (but not must) be initialized in the plugin init
  function. Doing it this way will avoid compiler warnings that
 "statical initializer has less values than the structure has fields".
- - handlerton init() function was used to return 1 (failure) is storage
  engine was disabled from command line. plugin init() function should
  return 0 (success) in this case.

3) New auto_increment interface (WL#3146)

- - The new auto_increment interface is now:

  virtual void get_auto_increment(ulonglong offset, ulonglong increment,
                                  ulonglong nb_desired_values,
                                  ulonglong *first_value,
                                  ulonglong *nb_reserved_values);
  virtual void release_auto_increment() { return; };
  virtual void restore_auto_increment();            (old)
  virtual int reset_auto_increment(ulonglong value) (old)

- - get_auto_increment() is used to tell engine that it should reserve
  a set of values for the table. The handler is free to reserve a
  smaller amount of values than what is requested.

  Here follows the documentation for get_auto_increment:

    nb_desired_values   how many values we want
    first_value         (OUT) the first value reserved by the handler
    nb_reserved_values  (OUT) how many values the handler reserved

  offset and increment means that we want values to be of the form
  offset + N * increment, where N>=0 is integer.
  If the function sets *first_value to ~(ulonglong)0 it means an error.
  If the function sets *nb_reserved_values to ULONGLONG_MAX it means it has
  reserved to "positive infinite".

- - release_auto_increment() is called to tell engine that we are done
  with query and it's free to release any not used auto_increment
  values for other usage (if it can do that).

NOTE: The handler interface for auto_increment is fully implemented in
      5.1, but we have not yet updated the statement logging code to
      log a set of intervals. This will be done shortly (in good time
      before 5.1 is released as release candidate).

4) Overall cleanup

Changes done that requires code changes in code of other storage engines:
(Note that all changes are very straightforward and one should find all issues
by compiling a --debug build and fixing all compiler errors and all
asserts in field.cc while running the test suite),

- - New optional handler function introduced: reset()
  This is called after every DML statement to make it easy for a handler to
  statement specific cleanups.
  (The only case it's not called is if force the file to be closed)

- - handler::extra(HA_EXTRA_RESET) is removed. Code that was there before
  should be moved to handler::reset()

- - table->read_set contains a bitmap over all columns that are needed
  in the query.  read_row() and similar functions only needs to read these
- - table->write_set contains a bitmap over all columns that will be updated
  in the query. write_row() and update_row() only needs to update these
  The above bitmaps should now be up to date in all context
  (including ALTER TABLE, filesort()).

  The handler is informed of any changes to the bitmap after
  fix_fields() by calling the virtual function
  handler::column_bitmaps_signal(). If the handler does caching of
  these bitmaps (instead of using table->read_set, table->write_set),
  it should redo the caching in this code. as the signal() may be sent
  several times, it's probably best to set a variable in the signal
  and redo the caching on read_row() / write_row() if the variable was

- - Removed the read_set and write_set bitmap objects from the handler class

- - Removed all column bit handling functions from the handler class.
  (Now one instead uses the normal bitmap functions in my_bitmap.c instead
  of handler dedicated bitmap functions)

- - field->query_id is removed. One should instead instead check
  table->read_set and table->write_set if a field is used in the query.

- - handler::extra(HA_EXTRA_RETRIVE_ALL_COLS) and
  handler::extra(HA_EXTRA_RETRIEVE_PRIMARY_KEY) are removed. One should now
  instead use table->read_set to check for which columns to retrieve.

- - If a handler needs to call Field->val() or Field->store() on columns
  that are not used in the query, one should install a temporary
  all-columns-used map while doing so. For this, we provide the following

  my_bitmap_map *old_map= dbug_tmp_use_all_columns(table, table->read_set);
  dbug_tmp_restore_column_map(table->read_set, old_map);

  and similar for the write map:

  my_bitmap_map *old_map= dbug_tmp_use_all_columns(table, table->write_set);
  dbug_tmp_restore_column_map(table->write_set, old_map);

  If this is not done, you will sooner or later hit a DBUG_ASSERT
  in the field store() / val() functions.
  (For not DBUG binaries, the dbug_tmp_restore_column_map() and
  dbug_tmp_restore_column_map() are inline dummy functions and should
  be optimized away be the compiler).

- - If one needs to temporary set the column map for all binaries (and not
  just to avoid the DBUG_ASSERT() in the Field::store() / Field::val()
  methods) one should use the functions tmp_use_all_columns() and
  tmp_restore_column_map() instead of the above dbug_ variants.

- - All 'status' fields in the handler base class (like records,
  data_file_length etc) are now stored in a 'stats' struct. This makes
  it easier to know what status variables are provided by the base
  handler.  This requires some trivial variable names in the extra()

- - New virtual function handler::records().  This is called to optimize
  COUNT(*) if (handler::table_flags() & HA_HAS_RECORDS()) is true.
  (stats.records is not supposed to be an exact value. It's only has to
  be 'reasonable enough' for the optimizer to be able to choose a good
  optimization path).

- - Non virtual handler::init() function added for caching of virtual
  constants from engine.

- - Removed has_transactions() virtual method. Now one should instead return
  HA_NO_TRANSACTIONS in table_flags() if the table handler DOES NOT support

- - The 'xxxx_create_handler()' function now has a MEM_ROOT_root argument
  that is to be used with 'new handler_name()' to allocate the handler
  in the right area.  The xxxx_create_handler() function is also
  responsible for any initialization of the object before returning.

  For example, one should change:

  static handler *myisam_create_handler(TABLE_SHARE *table)
    return new ha_myisam(table);


  static handler *myisam_create_handler(TABLE_SHARE *table, MEM_ROOT *mem_root)
    return new (mem_root) ha_myisam(table);

- - New optional virtual function: use_hidden_primary_key().
  This is called in case of an update/delete when
  (table_flags() and HA_PRIMARY_KEY_REQUIRED_FOR_DELETE) is defined
  but we don't have a primary key. This allows the handler to take precisions
  in remembering any hidden primary key to able to update/delete any
  found row. The default handler marks all columns to be read.

- - handler::table_flags() now returns a ulonglong (to allow for more flags).

- - New/changed table_flags()
  - HA_HAS_RECORDS          Set if ::records() is supported
  - HA_NO_TRANSACTIONS      Set if engine doesn't support transactions
                            Set if we should mark all primary key columns for
                            read when reading rows as part of a DELETE
                            statement. If there is no primary key,
                            all columns are marked for read.
  - HA_PARTIAL_COLUMN_READ  Set if engine will not read all columns in some
                            cases (based on table->read_set)
                            Renamed to HA_PRIMARY_KEY_REQUIRED_FOR_POSITION.
 - HA_DUPP_POS              Renamed to HA_DUPLICATE_POS
                            Set this if we should mark ALL key columns for
                            read when when reading rows as part of a DELETE
                            statement. In case of an update we will mark
                            all keys for read for which key part changed
                             Set this if stats.records is exact.
                             (This saves us some extra records() calls
                             when optimizing COUNT(*))

- - Removed table_flags()
  - HA_NOT_EXACT_COUNT     Now one should instead use HA_HAS_RECORDS if
                           handler::records() gives an exact count() and
                           HA_STATS_RECORDS_IS_EXACT if stats.records is exact.
  - HA_READ_RND_SAME       Removed (no one supported this one)

- - Removed not needed functions ha_retrieve_all_cols() and ha_retrieve_all_pk()

- - Renamed handler::dupp_pos to handler::dup_pos

- - Removed not used variable handler::sortkey

Upper level handler changes:

- - ha_reset() now does some overall checks and calls ::reset()
- - ha_table_flags() added. This is a cached version of table_flags(). The
  cache is updated on engine creation time and updated on open.

MySQL level changes (not obvious from the above):

- - DBUG_ASSERT() added to check that column usage matches what is set
  in the column usage bit maps. (This found a LOT of bugs in current
  column marking code).

- - In 5.1 before, all used columns was marked in read_set and only updated
  columns was marked in write_set. Now we only mark columns for which we
  need a value in read_set.

- - Column bitmaps are created in open_binary_frm() and open_table_from_share().
  (Before this was in table.cc)

- - handler::table_flags() calls are replaced with handler::ha_table_flags()

- - For calling field->val() you must have the corresponding bit set in
  table->read_set. For calling field->store() you must have the
  corresponding bit set in table->write_set. (There are asserts in
  all store()/val() functions to catch wrong usage)

- - thd->set_query_id is renamed to thd->mark_used_columns and instead
  of setting this to an integer value, this has now the values:
  Changed also all variables named 'set_query_id' to mark_used_columns.

- - In filesort() we now inform the handler of exactly which columns are needed
  doing the sort and choosing the rows.

- - The TABLE_SHARE object has a 'all_set' column bitmap one can use
  when one needs a column bitmap with all columns set.
  (This is used for table->use_all_columns() and other places)

- - The TABLE object has 3 column bitmaps:
  - def_read_set     Default bitmap for columns to be read
  - def_write_set    Default bitmap for columns to be written
  - tmp_set          Can be used as a temporary bitmap when needed.
  The table object has also two pointer to bitmaps read_set and write_set
  that the handler should use to find out which columns are used in which way.

- - count() optimization now calls handler::records() instead of using
  handler->stats.records (if (table_flags() & HA_HAS_RECORDS) is true).

- - Added extra argument to Item::walk() to indicate if we should also
  traverse sub queries.

- - Added TABLE parameter to cp_buffer_from_ref()

- - Don't close tables created with CREATE ... SELECT but keep them in
  the table cache. (Faster usage of newly created tables).

New interfaces:

- - table->clear_column_bitmaps() to initialize the bitmaps for tables
  at start of new statements.

- - table->column_bitmaps_set() to set up new column bitmaps and signal
  the handler about this.

- - table->column_bitmaps_set_no_signal() for some few cases where we need
  to setup new column bitmaps but don't signal the handler (as the handler
  has already been signaled about these before). Used for the momement
  only in opt_range.cc when doing ROR scans.

- - table->use_all_columns() to install a bitmap where all columns are marked
  as use in the read and the write set.

- - table->default_column_bitmaps() to install the normal read and write
  column bitmaps, but not signaling the handler about this.
  This is mainly used when creating TABLE instances.

- - table->mark_columns_needed_for_delete(),
  table->mark_columns_needed_for_delete() and
  table->mark_columns_needed_for_insert() to allow us to put additional
  columns in column usage maps if handler so requires.
  (The handler indicates what it neads in handler->table_flags())

- - table->prepare_for_position() to allow us to tell handler that it
  needs to read primary key parts to be able to store them in
  future table->position() calls.
  (This replaces the table->file->ha_retrieve_all_pk function)

- - table->mark_auto_increment_column() to tell handler are going to update
  columns part of any auto_increment key.

- - table->mark_columns_used_by_index() to mark all columns that is part of
  an index.  It will also send extra(HA_EXTRA_KEYREAD) to handler to allow
  it to quickly know that it only needs to read colums that are part
  of the key.  (The handler can also use the column map for detecting this,
  but simpler/faster handler can just monitor the extra() call).

- - table->mark_columns_used_by_index_no_reset() to in addition to other 
  also mark all columns that is used by the given key.

- - table->restore_column_maps_after_mark_index() to restore to default
  column maps after a call to table->mark_columns_used_by_index().

- - New item function register_field_in_read_map(), for marking used columns
  in table->read_map. Used by filesort() to mark all used columns

- - Maintain in TABLE->merge_keys set of all keys that are used in query.
  (Simplices some optimization loops)

- - Maintain Field->part_of_key_not_clustered which is like Field->part_of_key
  but the field in the clustered key is not assumed to be part of all index.
  (used in opt_range.cc for faster loops)

- -  dbug_tmp_use_all_columns(), dbug_tmp_restore_column_map()
   tmp_use_all_columns() and tmp_restore_column_map() functions to temporally
   mark all columns as usable.  The 'dbug_' version is primarily intended
   inside a handler when it wants to just call Field:store() & Field::val()
   functions, but don't need the column maps set for any other usage.
   (ie:: bitmap_is_set() is never called)

- - We can't use compare_records() to skip updates for handlers that returns
  a partial column set and the read_set doesn't cover all columns in the
  write set. The reason for this is that if we have a column marked only for
  write we can't in the MySQL level know if the value changed or not.
  The reason this worked before was that MySQL marked all to be written
  columns as also to be read. The new 'optimal' bitmaps exposed this 'hidden

- - open_table_from_share() does not anymore setup temporary MEM_ROOT
  object as a thread specific variable for the handler. Instead we
  send the to-be-used MEMROOT to get_new_handler().
  (Simpler, faster code)

Bugs fixed:

- - Column marking was not done correctly in a lot of cases.
  (ALTER TABLE, when using triggers, auto_increment fields etc)
  (Could potentially result in wrong values inserted in table handlers
  relying on that the old column maps or field->set_query_id was correct)
  Especially when it comes to triggers, there may be cases where the
  old code would cause lost/wrong values for NDB and/or InnoDB tables.

- - Split thd->options flag OPTION_STATUS_NO_TRANS_UPDATE to two flags:
  This allowed me to remove some wrong warnings about:
  "Some non-transactional changed tables couldn't be rolled back"

- - Fixed handling of INSERT .. SELECT and CREATE ... SELECT that wrongly reset
  (thd->options & OPTION_STATUS_NO_TRANS_UPDATE) which caused us to loose
  some warnings about
  "Some non-transactional changed tables couldn't be rolled back")

- - Fixed use of uninitialized memory in ha_ndbcluster.cc::delete_table()
  which could cause delete_table to report random failures.

- - Fixed core dumps for some tests when running with --debug

- - Added missing FN_LIBCHAR in mysql_rm_tmp_tables()
  (This has probably caused us to not properly remove temporary files after

- - slow_logs was not properly initialized, which could maybe cause
  extra/lost entries in slow log.

- - If we get an duplicate row on insert, change column map to read and
  write all columns while retrying the operation. This is required by
  the definition of REPLACE and also ensures that fields that are only
  part of UPDATE are properly handled.  This fixed a bug in NDB and
  REPLACE where REPLACE wrongly copied some column values from the replaced

- - For table handler that doesn't support NULL in keys, we would give an error
  when creating a primary key with NULL fields, even after the fields has been
  automaticly converted to NOT NULL.

- - Creating a primary key on a SPATIAL key, would fail if field was not
  declared as NOT NULL.


- - Removed not used condition argument to setup_tables

- - Removed not needed item function reset_query_id_processor().

- - Field->add_index is removed. Now this is instead maintained in
  (field->flags & FIELD_IN_ADD_INDEX)

- - Field->fieldnr is removed (use field->field_index instead)

- - New argument to filesort() to indicate that it should return a set of
  row pointers (not used columns). This allowed me to remove some references
  to sql_command in filesort and should also enable us to return column
  results in some cases where we couldn't before.

- - Changed column bitmap handling in opt_range.cc to be aligned with TABLE
  bitmap, which allowed me to use bitmap functions instead of looping over
  all fields to create some needed bitmaps. (Faster and smaller code)

- - Broke up found too long lines

- - Moved some variable declaration at start of function for better code

- - Removed some not used arguments from functions.
  (setup_fields(), mysql_prepare_insert_check_table())

- - setup_fields() now takes an enum instead of an int for marking columns

- - For internal temporary tables, use handler::write_row(),
  handler::delete_row() and handler::update_row() instead of
  handler::ha_xxxx() for faster execution.

- - Changed some constants to enum's and define's.

- - Using separate column read and write sets allows for easier checking
  of timestamp field was set by statement.

- - Remove calls to free_io_cache() as this is now done automaticly in 

- - Don't build table->normalized_path as this is now identical to table->path
  (after bar's fixes to convert filenames)

- - Fixed some missed DBUG_PRINT(.."%lx") to use "0x%lx" to make it easier to
  do comparision with the 'convert-dbug-for-diff' tool.

Things left to do in 5.1:

- - We wrongly log failed CREATE TABLE ... SELECT in some cases when using
  row based logging (as shown by testcase binlog_row_mix_innodb_myisam.result)
  Mats has promised to look into this.

- - Test that my fix for CREATE TABLE ... SELECT is indeed correct.
  (I added several test cases for this, but in this case it's better that
  someone else also tests this throughly).
  Lars has promosed to do this.

- - Update worklog #WL3281 with things that where not done in this cleanup.
  What is should still be done in 5.1 are:

 - Add a hash of all tables used by any active transaction. We need
   this to not drop or rename a table or database while an object is
   still in use.  (Currently we only detect if a table is in use by a
   running statement)

 - Maybe: New interface of index_read() to be able to handle bitmap indexes
   (Need to change length of key to bitmap to say which key parts are

- -------------


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