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Added comments to all methods. Added explanation for a sequential read through a storage engine. 


sql/examples/ha_example.cc:
  Documentation updates (lots of comments in the code).
sql/examples/ha_example.h:
  Documenation update, lots of comments in the code.
parent fabcf369
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sql/examples/ha_example.cc
View file @ 8351c87f
......@@ -14,6 +14,55 @@
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
/*
ha_example is a stubbed storage engine. It does nothing at this point. It
will let you create/open/delete tables but that is all. You can enable it
in your buld by doing the following during your build process:
./configure --with-example-storage-engine
Once this is done mysql will let you create tables with:
CREATE TABLE A (...) ENGINE=EXAMPLE;
The example is setup to use table locks. It implements an example "SHARE"
that is inserted into a hash by table name. You can use this to store
information of state that any example handler object will be able to see
if it is using the same table.
Please read the object definition in ha_example.h before reading the rest
if this file.
To get an idea of what occurs here is an example select that would do a
scan of an entire table:
ha_example::store_lock
ha_example::external_lock
ha_example::info
ha_example::rnd_init
ha_example::extra
ENUM HA_EXTRA_CACHE Cash record in HA_rrnd()
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::rnd_next
ha_example::extra
ENUM HA_EXTRA_NO_CACHE End cacheing of records (def)
ha_example::external_lock
ha_example::extra
ENUM HA_EXTRA_RESET Reset database to after open
In the above example has 9 row called before rnd_next signalled that it was
at the end of its data. In the above example the table was already opened
(or you would have seen a call to ha_example::open(). Calls to
ha_example::extra() are hints as to what will be occuring to the request.
Happy coding!
-Brian
*/
#ifdef __GNUC__
#pragma implementation // gcc: Class implementation
#endif
......@@ -24,10 +73,14 @@
#include "ha_example.h"
/* Variables for example share methods */
pthread_mutex_t example_mutex;
static HASH example_open_tables;
static int example_init= 0;
static HASH example_open_tables; // Hash used to track open tables
pthread_mutex_t example_mutex; // This is the mutex we use to init the hash
static int example_init= 0; // Variable for checking the init state of hash
/*
Function we use in the creation of our hash to get key.
*/
static byte* example_get_key(EXAMPLE_SHARE *share,uint *length,
my_bool not_used __attribute__((unused)))
{
......@@ -37,7 +90,9 @@ static byte* example_get_key(EXAMPLE_SHARE *share,uint *length,
/*
Example of simple lock controls.
Example of simple lock controls. The "share" it creates is structure we will
pass to each example handler. Do you have to have one of these? Well, you have
pieces that are used for locking, and they are needed to function.
*/
static EXAMPLE_SHARE *get_share(const char *table_name, TABLE *table)
{
......@@ -45,6 +100,12 @@ static EXAMPLE_SHARE *get_share(const char *table_name, TABLE *table)
uint length;
char *tmp_name;
/*
So why does this exist? There is no way currently to init a storage engine.
Innodb and BDB both have modifications to the server to allow them to
do this. Since you will not want to do this, this is probably the next
best method.
*/
if (!example_init)
{
/* Hijack a mutex for init'ing the storage engine */
......@@ -101,7 +162,8 @@ static EXAMPLE_SHARE *get_share(const char *table_name, TABLE *table)
/*
Free lock controls.
Free lock controls. We call this whenever we close a table. If the table had
the last reference to the share then we free memory associated with it.
*/
static int free_share(EXAMPLE_SHARE *share)
{
......@@ -119,10 +181,24 @@ static int free_share(EXAMPLE_SHARE *share)
}
/*
If frm_error() is called then we will use this to to find out what file extentions
exist for the storage engine. This is also used by the default rename_table and
delete_table method in handler.cc.
*/
const char **ha_example::bas_ext() const
{ static const char *ext[]= { NullS }; return ext; }
/*
Used for opening tables. The name will be the name of the file.
A table is opened when it needs to be opened. For instance
when a request comes in for a select on the table (tables are not
open and closed for each request, they are cached).
Called from handler.cc by handler::ha_open(). The server opens all tables by
calling ha_open() which then calls the handler specific open().
*/
int ha_example::open(const char *name, int mode, uint test_if_locked)
{
DBUG_ENTER("ha_example::open");
......@@ -134,18 +210,66 @@ int ha_example::open(const char *name, int mode, uint test_if_locked)
DBUG_RETURN(0);
}
/*
Closes a table. We call the free_share() function to free any resources
that we have allocated in the "shared" structure.
Called from sql_base.cc, sql_select.cc, and table.cc.
In sql_select.cc it is only used to close up temporary tables or during
the process where a temporary table is converted over to being a
myisam table.
For sql_base.cc look at close_data_tables().
*/
int ha_example::close(void)
{
DBUG_ENTER("ha_example::close");
DBUG_RETURN(free_share(share));
}
/*
write_row() inserts a row. No extra() hint is given currently if a bulk load
is happeneding. buf() is a byte array of data. You can use the field
information to extract the data from the native byte array type.
Example of this would be:
for (Field **field=table->field ; *field ; field++)
{
...
}
See ha_tina.cc for an example of extracting all of the data as strings.
ha_berekly.cc has an example of how to store it intact by "packing" it
for ha_berkeley's own native storage type.
See the note for update_row() on auto_increments and timestamps. This
case also applied to write_row().
Called from item_sum.cc, item_sum.cc, sql_acl.cc, sql_insert.cc,
sql_insert.cc, sql_select.cc, sql_table.cc, sql_udf.cc, and sql_update.cc.
*/
int ha_example::write_row(byte * buf)
{
DBUG_ENTER("ha_example::write_row");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
Yes, update_row() does what you expect, it updates a row. old_data will have
the previous row record in it, while new_data will have the newest data in
it.
Keep in mind that the server can do updates based on ordering if an ORDER BY
clause was used. Consecutive ordering is not guarenteed.
Currently new_data will not have an updated auto_increament record, or
and updated timestamp field. You can do these for example by doing these:
if (table->timestamp_on_update_now)
update_timestamp(new_row+table->timestamp_on_update_now-1);
if (table->next_number_field && record == table->record[0])
update_auto_increment();
Called from sql_select.cc, sql_acl.cc, sql_update.cc, and sql_insert.cc.
*/
int ha_example::update_row(const byte * old_data, byte * new_data)
{
......@@ -153,12 +277,32 @@ int ha_example::update_row(const byte * old_data, byte * new_data)
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
This will delete a row. buf will contain a copy of the row to be deleted.
The server will call this right after the current row has been called (from
either a previous rnd_nexT() or index call).
If you keep a pointer to the last row or can access a primary key it will
make doing the deletion quite a bit easier.
Keep in mind that the server does no guarentee consecutive deletions. ORDER BY
clauses can be used.
Called in sql_acl.cc and sql_udf.cc to manage internal table information.
Called in sql_delete.cc, sql_insert.cc, and sql_select.cc. In sql_select it is
used for removing duplicates while in insert it is used for REPLACE calls.
*/
int ha_example::delete_row(const byte * buf)
{
DBUG_ENTER("ha_example::delete_row");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
Positions an index cursor to the index specified in the handle. Fetches the
row if available. If the key value is null, begin at the first key of the
index.
*/
int ha_example::index_read(byte * buf, const byte * key,
uint key_len __attribute__((unused)),
enum ha_rkey_function find_flag
......@@ -168,6 +312,11 @@ int ha_example::index_read(byte * buf, const byte * key,
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
Positions an index cursor to the index specified in key. Fetches the
row if any. This is only used to read whole keys.
*/
int ha_example::index_read_idx(byte * buf, uint index, const byte * key,
uint key_len __attribute__((unused)),
enum ha_rkey_function find_flag
......@@ -178,66 +327,187 @@ int ha_example::index_read_idx(byte * buf, uint index, const byte * key,
}
/*
Used to read forward through the index.
*/
int ha_example::index_next(byte * buf)
{
DBUG_ENTER("ha_example::index_next");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
Used to read backwards through the index.
*/
int ha_example::index_prev(byte * buf)
{
DBUG_ENTER("ha_example::index_prev");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
index_first() asks for the first key in the index.
Called from opt_range.cc, opt_sum.cc, sql_handler.cc,
and sql_select.cc.
*/
int ha_example::index_first(byte * buf)
{
DBUG_ENTER("ha_example::index_first");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
index_last() asks for the last key in the index.
Called from opt_range.cc, opt_sum.cc, sql_handler.cc,
and sql_select.cc.
*/
int ha_example::index_last(byte * buf)
{
DBUG_ENTER("ha_example::index_last");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
rnd_init() is called when the system wants the storage engine to do a table
scan.
See the example in the introduction at the top of this file to see when
rnd_init() is called.
Called from filesort.cc, records.cc, sql_handler.cc, sql_select.cc, sql_table.cc,
and sql_update.cc.
*/
int ha_example::rnd_init(bool scan)
{
DBUG_ENTER("ha_example::rnd_init");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
This is called for each row of the table scan. When you run out of records
you should return HA_ERR_END_OF_FILE. Fill buff up with the row information.
The Field structure for the table is the key to getting data into buf
in a manner that will allow the server to understand it.
Called from filesort.cc, records.cc, sql_handler.cc, sql_select.cc, sql_table.cc,
and sql_update.cc.
*/
int ha_example::rnd_next(byte *buf)
{
DBUG_ENTER("ha_example::rnd_next");
DBUG_RETURN(HA_ERR_END_OF_FILE);
}
/*
position() is called after each call to rnd_next() if the data needs
to be ordered. You can do something like the following to store
the position:
ha_store_ptr(ref, ref_length, current_position);
The server uses ref to store data. ref_length in the above case is
the size needed to store current_position. ref is just a byte array
that the server will maintain. If you are using offsets to mark rows, then
current_position should be the offset. If it is a primary key like in
BDB, then it needs to be a primary key.
Called from filesort.cc, sql_select.cc, sql_delete.cc and sql_update.cc.
*/
void ha_example::position(const byte *record)
{
DBUG_ENTER("ha_example::position");
DBUG_VOID_RETURN;
}
/*
This is like rnd_next, but you are given a position to use
to determine the row. The position will be of the type that you stored in
ref. You can use ha_get_ptr(pos,ref_length) to retrieve whatever key
or position you saved when position() was called.
Called from filesort.cc records.cc sql_insert.cc sql_select.cc sql_update.cc.
*/
int ha_example::rnd_pos(byte * buf, byte *pos)
{
DBUG_ENTER("ha_example::rnd_pos");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
::info() is used to return information to the optimizer.
Currently this table handler doesn't implement most of the fields
really needed. SHOW also makes use of this data
Another note, you will probably want to have the following in your
code:
if (records < 2)
records = 2;
The reason is that the server will optimize for cases of only a single
record. If in a table scan you don't know the number of records
it will probably be better to set records to two so you can return
as many records as you need.
Along with records a few more variables you may wish to set are:
records
deleted
data_file_length
index_file_length
delete_length
check_time
Take a look at the public variables in handler.h for more information.
Called in:
filesort.cc
ha_heap.cc
item_sum.cc
opt_sum.cc
sql_delete.cc
sql_delete.cc
sql_derived.cc
sql_select.cc
sql_select.cc
sql_select.cc
sql_select.cc
sql_select.cc
sql_show.cc
sql_show.cc
sql_show.cc
sql_show.cc
sql_table.cc
sql_union.cc
sql_update.cc
*/
void ha_example::info(uint flag)
{
DBUG_ENTER("ha_example::info");
DBUG_VOID_RETURN;
}
/*
extra() is called whenever the server wishes to send a hint to
the storage engine. The myisam engine implements the most hints.
ha_innodb.cc has the most exhaustive list of these hints.
*/
int ha_example::extra(enum ha_extra_function operation)
{
DBUG_ENTER("ha_example::extra");
DBUG_RETURN(0);
}
/*
Deprecated and likely to be removed in the future. Storage engines normally
just make a call like:
ha_example::extra(HA_EXTRA_RESET);
to handle it.
*/
int ha_example::reset(void)
{
DBUG_ENTER("ha_example::reset");
......@@ -245,18 +515,71 @@ int ha_example::reset(void)
}
/*
Used to delete all rows in a table. Both for cases of truncate and
for cases where the optimizer realizes that all rows will be
removed as a result of a SQL statement.
Called from item_sum.cc by Item_func_group_concat::clear(),
Item_sum_count_distinct::clear(), and Item_func_group_concat::clear().
Called from sql_delete.cc by mysql_delete().
Called from sql_select.cc by JOIN::reinit().
Called from sql_union.cc by st_select_lex_unit::exec().
*/
int ha_example::delete_all_rows()
{
DBUG_ENTER("ha_example::delete_all_rows");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
First you should go read the section "locking functions for mysql" in
lock.cc to understand this.
This create a lock on the table. If you are implementing a storage engine
that can handle transacations look at ha_berkely.cc to see how you will
want to goo about doing this. Otherwise you should consider calling flock()
here.
Called from lock.cc by lock_external() and unlock_external(). Also called
from sql_table.cc by copy_data_between_tables().
*/
int ha_example::external_lock(THD *thd, int lock_type)
{
DBUG_ENTER("ha_example::external_lock");
DBUG_RETURN(0);
}
/*
The idea with handler::store_lock() is the following:
The statement decided which locks we should need for the table
for updates/deletes/inserts we get WRITE locks, for SELECT... we get
read locks.
Before adding the lock into the table lock handler (see thr_lock.c)
mysqld calls store lock with the requested locks. Store lock can now
modify a write lock to a read lock (or some other lock), ignore the
lock (if we don't want to use MySQL table locks at all) or add locks
for many tables (like we do when we are using a MERGE handler).
Berkeley DB for example changes all WRITE locks to TL_WRITE_ALLOW_WRITE
(which signals that we are doing WRITES, but we are still allowing other
reader's and writer's.
When releasing locks, store_lock() are also called. In this case one
usually doesn't have to do anything.
In some exceptional cases MySQL may send a request for a TL_IGNORE;
This means that we are requesting the same lock as last time and this
should also be ignored. (This may happen when someone does a flush
table when we have opened a part of the tables, in which case mysqld
closes and reopens the tables and tries to get the same locks at last
time). In the future we will probably try to remove this.
Called from lock.cc by get_lock_data().
*/
THR_LOCK_DATA **ha_example::store_lock(THD *thd,
THR_LOCK_DATA **to,
enum thr_lock_type lock_type)
......@@ -267,6 +590,16 @@ THR_LOCK_DATA **ha_example::store_lock(THD *thd,
return to;
}
/*
Used to delete a table. By the time delete_table() has been called all
opened references to this table will have been closed (and your globally
shared references released. The variable name will just be the name of
the table. You will need to remove any files you have created at this point.
Called from handler.cc by delete_table and ha_create_table(). Only used
during create if the table_flag HA_DROP_BEFORE_CREATE was specified for
the storage engine.
*/
int ha_example::delete_table(const char *name)
{
DBUG_ENTER("ha_example::delete_table");
......@@ -274,12 +607,24 @@ int ha_example::delete_table(const char *name)
DBUG_RETURN(0);
}
/*
Renames a table from one name to another from alter table call.
Called from sql_table.cc by mysql_rename_table().
*/
int ha_example::rename_table(const char * from, const char * to)
{
DBUG_ENTER("ha_example::rename_table ");
DBUG_RETURN(HA_ERR_NOT_IMPLEMENTED);
}
/*
Given a starting key, and an ending key estimate the number of rows that
will exist between the two. end_key may be empty which in case determine
if start_key matches any rows.
Called from opt_range.cc by check_quick_keys().
*/
ha_rows ha_example::records_in_range(int inx,
const byte *start_key,uint start_key_len,
enum ha_rkey_function start_search_flag,
......@@ -287,11 +632,22 @@ ha_rows ha_example::records_in_range(int inx,
enum ha_rkey_function end_search_flag)
{
DBUG_ENTER("ha_example::records_in_range ");
DBUG_RETURN(records); // HA_ERR_NOT_IMPLEMENTED
DBUG_RETURN(records);
}
int ha_example::create(const char *name, TABLE *table_arg, HA_CREATE_INFO *create_info)
/*
create() is called to create a database. The variable name will have the name
of the table. When create() is called you do not need to worry about opening
the table. Also, the FRM file will have already been created so adjusting
create_info will not do you any good. You can overwrite the frm file at this
point if you wish to change the table definition, but there are no methods
currently provided for doing that.
Called from handle.cc by ha_create_table().
*/
int ha_example::create(const char *name, TABLE *table_arg,
HA_CREATE_INFO *create_info)
{
DBUG_ENTER("ha_example::create");
/* This is not implemented but we want someone to be able that it works. */
......
sql/examples/ha_example.h
View file @ 8351c87f
......@@ -14,6 +14,17 @@
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
/*
Please read ha_exmple.cc before reading this file.
Please keep in mind that the example storage engine implements all methods
that are required to be implemented. handler.h has a full list of methods
that you can implement.
*/
/*
EXAMPLE_SHARE is a structure that will be shared amoung all open handlers
The example implements the minimum of what you will probably need.
*/
typedef struct st_example_share {
char *table_name;
uint table_name_length,use_count;
......@@ -21,6 +32,9 @@ typedef struct st_example_share {
THR_LOCK lock;
} EXAMPLE_SHARE;
/*
Class definition for the storage engine
*/
class ha_example: public handler
{
THR_LOCK_DATA lock; /* MySQL lock */
......@@ -33,17 +47,34 @@ class ha_example: public handler
~ha_example()
{
}
const char *table_type() const { return "EXAMPLE"; }
/* The name that will be used for display purposes */
const char *table_type() const { return "EXAMPLE"; }
/* The name of the index type that will be used for display */
const char *index_type(uint inx) { return "NONE"; }
const char **bas_ext() const;
/*
This is a list of flags that says what the storage engine
implements. The current table flags are documented in
table_flags.
*/
ulong table_flags() const
{
return 0;
}
/*
This is a list of flags that says how the storage engine
implements indexes. The current index flags are documented in
handler.h. If you do not implement indexes, just return zero
here.
*/
ulong index_flags(uint inx) const
{
return 0;
}
/*
unireg.cc will call the following to make sure that the storage engine can
handle the data it is about to send.
*/
uint max_record_length() const { return HA_MAX_REC_LENGTH; }
uint max_keys() const { return 0; }
uint max_key_parts() const { return 0; }
......@@ -52,10 +83,15 @@ class ha_example: public handler
Called in test_quick_select to determine if indexes should be used.
*/
virtual double scan_time() { return (double) (records+deleted) / 20.0+10; }
/* The next method will never be called */
/*
The next method will never be called if you do not implement indexes.
*/
virtual double read_time(ha_rows rows) { return (double) rows / 20.0+1; }
virtual bool fast_key_read() { return 1;}
/*
Everything below are methods that we implment in ha_example.cc.
*/
int open(const char *name, int mode, uint test_if_locked);
int close(void);
int write_row(byte * buf);
......
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