HOLD_CURSOR, and RELEASE_CURSOR from the default values before precompiling the application
我们说的游标概念比较复杂，它可以是客户端程序中的游标，服务进程中的私有游标，以及服务器端共享池里的共享游标。假设一个游标被打开了，一般来说它的共享游标信息(包括执行计划，优化树等)总是会在SQL AREA里，无需再次软/硬解析。
参数文件中的连个游标相关的值：
SQL> show parameter CURSORS
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
open_cursors integer300
session_cached_cursors integer50
open_cursors：指明每个session最多能够打开的cursors的数量。
SESSION_CACHED_CURSORS：是Oracle中的一个初始化参数(修改必须重启实例)，指定了每个会话缓存的游标上限(保留在PGA中)；客户端程序中open cursor的要求仍会被传递给服务进程，服务进程首先扫描自身缓存的游标信息，如果命中则可以避免软解析，也有人称它为“软软解析”。
如果Open_cursors设置太小，对系统性能不会有明显改善，还可能触发ORA-O1000：m~imum open CUrsOrs exceeded．的错误。如果设置太大，则无端消耗系统内存。我们可以通过如下的sql语句查看你的设置是否合理：
SELECT MAX(A.VALUE) AS HIGHEST_OPEN_CUR, P.VALUE AS MAX_OPEN_CUR
FROM V$SESSTAT A, V$STATNAME B, V$PARAMETER P
WHERE A.STATISTIC# = B.STATISTIC#
AND B.NAME = 'opened cursors current'
AND P.NAME = 'open_cursors'
GROUP BY P.VALUE;
HIGHEST_ OPEN CUR是实际打开的cursors 的最大值，MAX_OPEN_ CUR是参数Open_cursors的设定值，如果二者太接近，甚至触发eRA一01000错误，那么你就应该调大参数Open_cursors的设定值。
HOLD_CURSOR：是预编译程序中的一个参数，它指定了私有游标是否应该该被缓存，这里不做展开。
在分析工具tkprof中hard parse与soft parse被同等对待，都被认为是parse；软解析即会造成parse总数上升。
软解析避免了硬解析过程中的几个步骤，但仍包括了初始化的语法，语义解析并计算语句HASH值与SQL AREA中已有语句进行对比；若匹配则查询优化等昂贵的操作得以避免。
另请注意，10053事件仅在硬解析过程中被触发。
What are the Oracle Precompiler options HOLD_CURSOR and RELEASE_CURSOR and how do they affect the performance of a PCC program?First of all, these options apply to implicit and explicit cursors in all precompiled languages except Pro*Ada. They apply ONLY to implicit cursors associated with INSERT, UPDATE, DELETE, or a single row SELECT in Pro*Ada. An explicit cursor in a program is one explicitly created with EXEC SQL DECLARE C1 CURSOR FOR...
This bulletin discusses what these options do internally and how changing them affects program performance.
HOLD_CURSOR is a precompiler command line parameter.
Summary Operation:
~~~~~~~~~~~~~~~~~~
NB: Both the soft and hard parse register as a parse in tkprof. If the cursor is open and it is not in the SQL_AREA then it clearly has to parse it (not shown in the diagram!)
The HOLD_CURSOR and RELEASE_CURSOR Options
------------------------------------------
.
Following will be a discussion of what these options do internally and how changing them affects program performance.
What exactly do we mean by CURSOR? Unfortunately, we mean two different things:
1. The program cursor - A data structure associated with a SQL statement.
A program cursor is declared for each SQL statement that the precompiler finds in your program. For the statements
EXEC SQL DECLARE SEL_EMP_CURS CURSOR FOR...
EXEC SQL INSERT...
PCC will declare two program cursors, say c1 and c2.
2. The Oracle cursor (also called the context area) - The workarea created dynamically at run time;
this area contains the parsed statement, the addresses of the host variables, and other information necessary to execute the SQL statement.
These two cursors are linked together via the cursor cache. The initial size of the cursor cache is determined by the MAXOPENCURSORS option.
The following diagram illustrates the relationship described above after an insert and an update have been executed in your program:
How are the HOLD_CURSOR and RELEASE_CURSOR options related to this view?
The HOLD_CURSOR option deals with the link between the program cursor and its cache entry.
The RELEASE_CURSOR option deals with the link between the Oracle cursor and the cache entry.
For SQL statements that are FREQUENTLY RE-EXECUTED, the bottom line is this: if you want to maximize performance, make sure these SQL statements stay "glued" to their respective Oracle cursor.
What does it mean when a SQL statement is "glued" to its Oracle cursor?
It means that both links between the SQL statement and its Oracle cursor are made permanent.
Why would you want to keep a statement "glued" to its context area?
Because the context area contains the parsed statement and other information necessary to execute the statement, such as the addresses of the host variables. Maintaining access to this information makes subsequent execution of the statement much faster.
How do you "glue" a statement to a cache entry?
By correct use of the HOLD_CURSOR and RELEASE_CURSOR options via the PCC command line or inline with EXEC ORACLE OPTION(...).
For instance, with HOLD_CURSOR=YES as the Oracle option, a cache entry cannot be flagged for reuse. This has important implications. If all cache entries have been used up and a new cache entry is needed for a new SQL statement such that the number of cache entries would now exceed the number specified in MAXOPENCURSORS, Oracle will use the first cache entry marked reuseable.
For example, in the above diagram, if the cache entry C(1) is marked reusable, and the program is about to execute the EXEC SQL SELECT...
(program cursor P(MAXOPENCURSORS+1), and the number of cache entries in use already equals MAXOPENCURSORS, cache entry C(1) and its Oracle
cursor will now be linked to the select statement. A subsequent execution of the insert statement would require pre-empting a cache entry and its Oracle cursor from another SQL statement and performing a reparse.
Correspondingly（相应的）, with the default RELEASE_CURSOR=NO as the Oracle option, the link between the cache entry and the Oracle cursor (the con-
text area) is maintained after the statement is executed so that the parsed statement and, more importantly, the allocated memory stay available.
The freeing up of this memory by RELEASE_CURSOR=YES means that the next statement that gets linked to this cache entry will require an expensive reallocation of memory in addition to a reparse. Ugh! Why would anybody want RELEASE_CURSOR=YES? We will see later on.
Program cursor - - - - - [ Cursor cache entry ] - - - - - Oracle for SQL statement cursor
^ ^
HOLD_CURSOR=YES RELEASE_CURSOR=NO
program cursor is permanently cache entry maintains the
linked to its cache entry. address of its context area.
So the HOLD_CURSOR option is intimately tied to the MAXOPENCURSORS option. What exactly is the MAXOPENCURSORS option? First of all, MAX-
OPENCURSORS is a misnomer（用词不当）. It should more appropriately be called INITIAL_CURSOR_CACHE_SIZE. (Okay, so it's a mouthful.) Anyway, if
all cursor cache entries are currently marked "not reusable" either because of the HOLD_CURSOR option or because the associated statement is currently being executed (an explicitly opened cursor is still being fetched on and hasn't been closed yet), then a request for a new cursor will actually result in the extension of the cursor cache at runtime (i.e. if MAXOPENCURSORS=10, and all 10 entries are active,then an 11th will be created).
Just letting the precompiler reuse the oldest cache entry won't always work, as the following example illustrates: Imagine the case where the user has ten explicitly declared cursors opened, and wants to execute an eleventh. If the program actually reuses the oldest program cursor, the user would lose his current position in the first cursor and would not be able to fetch from it anymore.
By the way, if an eleventh cache entry is created, when that cursor is closed the eleventh entry is not removed. Setting MAXOPENCURSORS low saves memory, but causes potentially expensive dynamic allocations of new cache entries if they're needed. Setting it high assures quick execution, but may use more memory than necessary.
What if a statement is not executed repeatedly in a program?
Then you could go with the other options HOLD_CURSOR=NO and RELEASE_CURSOR=YES. With the HOLD_CURSOR=NO option, the link between a program cursor and its cache entry is not permanent. The cache entry is automatically marked reusable in case it is needed. With the RELEASE_CURSOR=YES option, the Oracle cursor (the context area) is automatically freed and the parsed statement lost. A reason you might use this option is if you are limited by the number of Oracle cursors (MAXOPENCURSORS) at your site due to memory issues. You may want to incur the cost of reallocating memory and reparsing in order to manage memory more effectively.
An advantage of setting RELEASE_CURSOR=YES is that until the link be-
tween the cache entry and the Oracle cursor (context area) is removed,
ORACLE keeps parse locks on any tables referenced in the SQL state-
ment. These parse locks prevent other users and you from ALTERing or
DROPping the tables (does ORA-0057 sound familiar?). Also, in Version
5, it will free up the read-consistent image of the referenced tables
stored in ORACLE's Before Image file.
What do we mean when we say that RELEASE_CURSOR=YES takes precedence
over HOLD_CURSOR=YES? With RELEASE_CURSOR=YES, the link between the
Oracle cursor and the cache entry is cut and the Oracle cursor is
freed (closed), so even if your program cursor is permanently linked
to the cache entry because HOLD_CURSOR=YES, you will still have to re-
allocate memory and reparse the statement. So subsequent executions
of a statement don't benefit from the HOLD_CURSOR=YES option because
RELEASE_CURSOR=YES.
For programmers experienced with OCI, here's the OCI equivalent of
what's happening:
#define MAXOPENCURSORS 5
char *sql_stmts[10];
curs_def cursor[MAXOPENCURSORS];
oopen(cursor[0],...);
osql3(cursor[0],...,sql_stmts[0],...);
An example of a "cache entry" being linked to another SQL statement
later on in the program is as follows:
osql3(cursor[0],...,sql_stmts[5],...);
I am forced to reuse one of my "cache entries" to execute the sixth
SQL statement.
An example of a context area being freed is:
oclose(cursor[0]);
Reusing cursor[0] would require another oopen() and another osql3()--
another dynamic allocation of memory and another reparse.
Conclusion
----------
As a programmer, you will get the most from these options by using
them selectively inline rather than specifying them as options at pre-
compile time.
Reference:
Pro*C/C++ Precompiler Programmer's Guide
