get_bulk.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: get_bulk.so,v 10.7 2004/09/03 19:47:57 mjc Exp $])

define(BDB_LOC, ..)

m4_ref_title(Access Methods,
    Retrieving records in bulk, @retrieving records in bulk,
    am_misc/align, am_misc/partial)

m4_p([dnl
When retrieving large numbers of records from the database, the number
of method calls can often dominate performance.  m4_db offers bulk get
interfaces which can significantly increase performance for some
applications.  To retrieve records in bulk, an application buffer must
be specified to the m4_ref(dbh_get) or m4_refT(dbc_get)s.  This is done
in the C API by setting the m4_arg(data) and m4_arg(ulen) fields of the
m4_arg(data) m4_ref(Dbt) to reference an application buffer, and the
m4_arg(flags) field of that structure to m4_ref(DB_DBT_USERMEM).  In
the m4_db C++ and Java APIs, the actions are similar, although there
are API-specific methods to set the m4_ref(Dbt) values.  Then, the
m4_ref(DB_MULTIPLE) or m4_ref(DB_MULTIPLE_KEY) flags are specified to
the m4_ref(dbh_get) or m4_refT(dbc_get)s, which cause multiple records
to be returned in the specified buffer.])

m4_p([dnl
The difference between m4_ref(DB_MULTIPLE) and m4_ref(DB_MULTIPLE_KEY)
is as follows: m4_ref(DB_MULTIPLE) returns multiple data items for a
single key.  For example, the m4_ref(DB_MULTIPLE) flag would be used to
retrieve all of the duplicate data items for a single key in a single
call.  The m4_ref(DB_MULTIPLE_KEY) flag is used to retrieve multiple
key/data pairs, where each returned key may or may not have duplicate
data items.])

m4_p([dnl
Once the m4_ref(dbh_get) or m4_refT(dbc_get) has returned, the
application will walk through the buffer handling the returned records.
This is implemented for the C and C++ APIs using four macros:
m4_ref(DB_MULTIPLE_INIT), m4_ref(DB_MULTIPLE_NEXT),
m4_ref(DB_MULTIPLE_KEY_NEXT), and m4_ref(DB_MULTIPLE_RECNO_NEXT).  For
the Java API, this is implemented as three iterator classes:
m4_linkjavadoc(com.sleepycat.db, MultipleDataEntry),
m4_linkjavadoc(com.sleepycat.db, MultipleKeyDataEntry), and
m4_linkjavadoc(com.sleepycat.db, MultipleRecnoDataEntry).])

m4_p([dnl
The m4_ref(DB_MULTIPLE_INIT) macro is always called first.  It
initializes a local application variable and the m4_arg(data)
m4_ref(Dbt) for stepping through the set of returned records.  Then,
the application calls one of the remaining three macros:
m4_ref(DB_MULTIPLE_NEXT), m4_ref(DB_MULTIPLE_KEY_NEXT), and
m4_ref(DB_MULTIPLE_RECNO_NEXT).])

m4_p([dnl
If the m4_ref(DB_MULTIPLE) flag was specified to the m4_ref(dbh_get) or
m4_refT(dbc_get), the application will always call the
m4_ref(DB_MULTIPLE_NEXT) macro.  If the m4_ref(DB_MULTIPLE_KEY) flag
was specified to the m4_ref(dbh_get) or m4_refT(dbc_get), and, the
underlying database is a Btree or Hash database, the application will
always call the m4_ref(DB_MULTIPLE_KEY_NEXT) macro.  If the
m4_ref(DB_MULTIPLE_KEY) flag was specified to the m4_ref(dbh_get) or
m4_refT(dbc_get), and, the underlying database is a Queue or Recno
database, the application will always call the
m4_ref(DB_MULTIPLE_RECNO_NEXT) macro.  The m4_ref(DB_MULTIPLE_NEXT),
m4_ref(DB_MULTIPLE_KEY_NEXT), and m4_ref(DB_MULTIPLE_RECNO_NEXT) macros
are called repeatedly, until the end of the returned records is reached.
The end of the returned records is detected by the application's local
pointer variable being set to NULL.])

m4_p([dnl
The following is an example of a routine that displays the contents of
a Btree database using the bulk return interfaces.])

include(ref/am_misc/get_bulk.cs)

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