PL/SQL 101: Understanding Ref Cursors
(update from the original thread: PL/SQL 101 : Understanding Ref Cursors)
Author: BluShadow
Last Updated: 22nd May 2015
Introduction
Often on the community we get questions along the lines of:
"I'm passing a ref cursor back from a function, how do I use the data in that ref cursor in my select statement/query"
or
"How can I tell how many rows are in my ref cursor"
Equally as often, the problem comes from a lack of understanding what a ref cursor actually is. So I thought I'd describe the basics behind ref cursors with a view to giving a clearer understanding of them.
All of the below examples are done through the command line of SQL*Plus. If you're using a different tool, some of the commands may differ and you should refer to the documentation for that product (or just learn to use SQL*Plus as it's the Best! ). Also, I've provided the main code without the SQL*Plus prompt or line numbers so it can just be copied and pasted easily.
1. What is a ref cursor and how is the data fetched
Let's start with a simple function that opens a ref cursor and returns it. (We'll just select some employee details for a supplied department)
create or replace function get_dept_emps(p_deptno in number) return sys_refcursor is
v_rc sys_refcursor;
begin
open v_rc for 'select empno, ename, mgr, sal from emp where deptno = :deptno' using p_deptno;
return v_rc;
end;
/
Function created.
Now, if we look at using this through SQL*Plus we first create ourselves a ref cursor variable to accept the results of the function (this is the equivalent of the PL/SQL sys_refcursor datatype), and then call the function to get the ref cursor back..
SQL> var rc refcursor
SQL> exec :rc := get_dept_emps(10);
PL/SQL procedure successfully completed.
Ok, so our variable "rc" has our ref cursor.
If we use the SQL*Plus 'print' command on our ref cursor we get..
SQL> print rc;
EMPNO ENAME MGR SAL
---------- ---------- ---------- ----------
7782 CLARK 7839 2450
7839 KING 5000
7934 MILLER 7782 1300
Brilliant, so our ref cursor has provided the rows we wanted from the employee table.
I'm so pleased with that, I want to do it again..
SQL> print rc;
SP2-0625: Error printing variable "rc"
Uh oh! What's happened here? Why can't we print the data that's in our ref cursor again?
A common mistake is that people believe that a ref cursor actually contains the result data from the query. The truth is that the ref cursor doesn't contain any data at all, it's just a pointer to the query.
So why did the first print statement print out the results?
SQL*Plus looked at the ref cursor and saw that it was an open cursor. As such it went into a loop, fetching each row of data from the database, using the ref cursor as it's reference (pointer) to the query, and displaying each row of data until it had no more rows to fetch. Once it's fetched all the rows it closes the cursor. The power of SQL*Plus's "print" command eh!
When we tried to print the ref cursor a second time, we got an error because SQL*Plus looked at the cursor, saw it was not an open cursor (the previous print call had closed it when it had finished) and couldn't perform the task of printing anything.
Let's look at this in a bit more detail so it's a little clearer. We'll use some PL/SQL code to break down the processing that SQL*Plus was doing internally...
Firstly, let's get the open cursor and ask it how many rows it has..
declare
v_rc sys_refcursor;
begin
v_rc := get_dept_emps(10); -- This returns an open cursor
dbms_output.put_line('Rows: '||v_rc%ROWCOUNT);
close v_rc;
end;
/
Rows: 0
PL/SQL procedure successfully completed.
Yep, sure enough it's reporting 0 rows.
It's important that we remember that ROWCOUNT reports how many rows have been fetched through a cursor. Just after opening the cursor we haven't fetched any rows yet. If we fetch a row of data then we can see this change..
declare
v_rc sys_refcursor;
v_empno number;
v_ename varchar2(10);
v_mgr number;
v_sal number;
begin
v_rc := get_dept_emps(10); -- This returns an open cursor
dbms_output.put_line('Pre Fetch: Rows: '||v_rc%ROWCOUNT);
fetch v_rc into v_empno, v_ename, v_mgr, v_sal;
dbms_output.put_line('Post Fetch: Rows: '||v_rc%ROWCOUNT);
close v_rc;
end;
/
Pre Fetch: Rows: 0
Post Fetch: Rows: 1
PL/SQL procedure successfully completed.
Ok, so clearly the fetch has changed the row count. That proves it.
Let's fetch all our data and display it..
declare
v_rc sys_refcursor;
v_empno number;
v_ename varchar2(10);
v_mgr number;
v_sal number;
begin
v_rc := get_dept_emps(10); -- This returns an open cursor
loop
fetch v_rc into v_empno, v_ename, v_mgr, v_sal;
exit when v_rc%NOTFOUND; -- Exit the loop when we've run out of data
dbms_output.put_line('Row: '||v_rc%ROWCOUNT||' # '||v_empno||','||v_ename||','||v_mgr||','||v_sal);
end loop;
close v_rc;
end;
/
Row: 1 # 7782,CLARK,7839,2450
Row: 2 # 7839,KING,,5000
Row: 3 # 7934,MILLER,7782,1300
PL/SQL procedure successfully completed.
And what happens if we try and fetch more data after it's finished, just like we tried to do in SQL*Plus...
declare
v_rc sys_refcursor;
v_empno number;
v_ename varchar2(10);
v_mgr number;
v_sal number;
begin
v_rc := get_dept_emps(10); -- This returns an open cursor
loop
fetch v_rc into v_empno, v_ename, v_mgr, v_sal;
exit when v_rc%NOTFOUND; -- Exit the loop when we've run out of data
dbms_output.put_line('Row: '||v_rc%ROWCOUNT||' # '||v_empno||','||v_ename||','||v_mgr||','||v_sal);
end loop;
close v_rc;
fetch v_rc into v_empno, v_ename, v_mgr, v_sal;
end;
/
Row: 1 # 7782,CLARK,7839,2450
Row: 2 # 7839,KING,,5000
Row: 3 # 7934,MILLER,7782,1300
declare
*
ERROR at line 1:
ORA-01001: invalid cursor
ORA-06512: at line 15
As expected we get an error. In SQL*Plus it captured that error and printed it's own 'friendly' response... 'Error printing variable "rc"'
So now we understand the basics of what a ref cursor is.
You can clearly see that it is just a pointer to the query and it doesn't contain any data itself, it just allows us to reference the query so that we can fetch data as we require it.
2. How can we use a ref cursor in a SQL query?
Ok, so now you've got your ref cursor you want to use it. But how can you fetch the data from the ref cursor inside another SQL Select statement?
Perhaps we can just select from it?
SQL> select * from get_dept_emps(10);
select * from get_dept_emps(10)
*
ERROR at line 1:
ORA-00933: SQL command not properly ended
Nope. How about if we tell SQL to treat it as a table?
SQL> select * from table(get_dept_emps(10));
select * from table(get_dept_emps(10))
*
ERROR at line 1:
ORA-22905: cannot access rows from a non-nested table item
What about using it as a set of data in an IN condition?
SQL> select * from emp where empno in (get_dept_emps(10));
select * from emp where empno in (get_dept_emps(10))
*
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected - got CURSER
(N.B. The spelling error of "CURSER" is Oracle's, not mine. )
(Ok, I know the ref cursor is referencing more than just the empno in that example, but it would still result in the same error if it just referenced the empno)
The problem we're having is because the ref cursor isn't a table of data either in the literal database sense or in an array/collection sense, and it's not a set of data that can be compared with the IN clause.
So what's the point in these ref cursors? Is there a way we can use them?
Yes there is..
First let's create a type structure on the database. Remember, SQL cannot access PL/SQL table structures (certainly prior to 12c) so the type must be a database object..
create or replace type emptype as object(empno number,
ename varchar2(10),
mgr number,
sal number);
/
Type created.
create or replace type t_emptype as table of emptype;
/
Type created.
Ok, so we have a structure to hold a record and a type that is a table of that structure. So far so good. But in order to populate that structure with data coming from the ref cursor we can't just select from it as we saw above. Instead we need to provide some PL/SQL to actually do the fetching of data for us and populate the structure...
create or replace function populate_emps(deptno in number := null) return t_emptype is
v_emptype t_emptype := t_emptype(); -- Declare a local table structure and initialize it
v_cnt number := 0;
v_rc sys_refcursor;
v_empno number;
v_ename varchar2(10);
v_mgr number;
v_sal number;
begin
v_rc := get_dept_emps(deptno);
loop
fetch v_rc into v_empno, v_ename, v_mgr, v_sal;
exit when v_rc%NOTFOUND;
v_emptype.extend;
v_cnt := v_cnt + 1;
v_emptype(v_cnt) := emptype(v_empno, v_ename, v_mgr, v_sal);
end loop;
close v_rc;
return v_emptype;
end;
/
Function created.
The above function calls the function that opens the ref cursor, then loops through, fetching each row and populating our SQL type structure. When all rows have been fetched, the ref cursor is closed and the SQL table type structure is passed back from the function.
So now we have something in an structure that SQL understands, we should be able to query directly from it..
select * from table(populate_emps(30));
EMPNO ENAME MGR SAL
---------- ---------- ---------- ----------
7499 ALLEN 7698 1600
7521 WARD 7698 1250
7654 MARTIN 7698 1250
7698 BLAKE 7839 2850
7844 TURNER 7698 1500
7900 JAMES 7698 950
6 rows selected.
select * from emp where empno in (select empno from table(populate_emps(30)));
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ----------- ---------- ---------- ----------
7499 ALLEN SALESMAN 7698 20-FEB-1981 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-1981 1250 500 30
7654 MARTIN SALESMAN 7698 28-SEP-1981 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-1981 2850 30
7844 TURNER SALESMAN 7698 08-SEP-1981 1500 0 30
7900 JAMES CLERK 7698 03-DEC-1981 950 30
6 rows selected.
Hoorah!
We've successfully taken our ref cursor (pointer) and used it to fetch the data back that we want in a structure that SQL can understand. Ok, the examples are pretty meaningless as they stand as we could easily have achieved the same results through a basic select, but the method is what is important to understand here.
3. What is the point of ref cursors?
A good question. Many people learn about ref cursors and then try and use them everywhere. Although I personally know how to write and use ref cursors, I've found very little use for them in my production code. If you are thinking of using a ref cursor, the first thing you should ask yourself is "Why do I need one?"
If your answer is that you need to write your code to dynamically generate SQL, then perhaps you should look at what you are trying to achieve. Typically dynamic SQL, in a lot of cases, is unnecessary and a major cause of unmaintainable and unscalable code as well as possible performance issues and SQL injection; not to mention that a well designed database would mean that you should already know your tables and columns and have no need for anything to be dynamic.
If you really have a valid need to dynamically create SQL and you fully understand the implications and risks involved, then a ref cursor is useful for this task. What you will find however is that you are limited with ref cursors to a fixed result structure, so it may not be as generic a solution as you had initially planned for. If you consider the examples from above, you will see that we had to define a SQL object/structure of known columns in order to receive the data from the ref cursor. So, whilst the function that opens the ref cursor could do so for any piece of query text, the only way of successfully getting the data out of that ref cursor is to know what columns we are expecting back from it.
A common reason we see on the community is people who are trying to take an unknown number of rows and pivot that data into a dynamically unknown number of columns. You may think that's a good reason to use dynamic queries, but first consider that, if you're trying to do this, you're requirement is to try and display the data in a user friendly way, and SQL isn't necessarily the right tool for this; the right tool being reporting tools, most of which can already take data from a standard SQL statement and pivot it dynamically for you.
There is, however, a way around the issue of not knowing column names because of a dynamic query.
Either use the DBMS_SQL package to dynamically generate and process your queries or, from 11g onwards, take your ref cursor and convert it to a DBMS_SQL cursor using 11g's new DBMS_SQL.TO_CURSOR_NUMBER feature.
DBMS_SQL is a very powerful way of writing dynamic SQL, and I'll demonstrate that in the section below.
Ref cursors are typically consumed by 3rd party interfaces, such as Java or .NET etc. which can describe the cursor to determine it's columns and datatypes. This can be done in PL/SQL (from 11g onwards) using DBMS_SQL...but there is rarely a good reason for doing it.
4. Processing a ref cursor dynamically (Advanced topic)
For completeness I'm going to demonstrate the processing of a ref cursor (though remember, you'd have to have a really good reason to want to do this).
In SQL*Plus, the interface hides the fact that, when it gets a ref cursor returned and you issue a 'print' command, it internally obtains a description of the ref cursor so that it can determine what columns there are, what they're called and what their datatypes are. It needs that information so that it can format the output it gives you.
This is similar in other 3rd party interfaces, whether it's Java, .NET or something else that supports ref cursors. They get the ref cursor returned as a pointer and can then obtain a description of it to know what it's going to provide.
However, in PL/SQL's native cursor processing, we cannot do this. When you issue a FETCH statement in PL/SQL, you are expected to fetch the data INTO a variable or structure that is appropriate for holding the results. To do that, you need to know the structure at the time you compile the code so that you can declare the appropriate variables or types.
As mentioned in the above section, from 11g onwards, Oracle does provide an additional function in the DBMS_SQL package called DBMS_SQL.TO_CURSOR_NUMBER. This function takes a ref cursor (remember it's just a pointer) and casts it to a DBMS_SQL cursor (which is another kind of pointer, actually just a number that references the cursor). Once it's a DBMS_SQL cursor, we have all the functionality that DBMS_SQL provides, which includes being able to DESCRIBE the cursor to find out it's columns and datatypes and to fetch the data based on column position rather than name (as we potentially don't know the names if it's dynamic!)...
What we're going to do here then, is to emulate (in a basic sense) what happens behind the scenes of the SQL*Plus 'print' command and other similar interfaces.
Let's start with a procedure that accepts a ref cursor as a parameter, converts it to a DBMS_SQL cursor, and then DESCRIBE's what the cursor results will look like:
create or replace procedure process_ref_cur(p_rc in out sys_refcursor) as
dbms_cur integer;
rec_tab dbms_sql.desc_tab2;
col_cnt integer;
begin
-- Convert the incoming Ref Cursor to a DBMS_SQL cursor
dbms_cur := dbms_sql.to_cursor_number(p_rc);
-- Get the description of the cursor projection.
-- This tells us the number of columns and a structured
-- datatype containing information about each column.
dbms_sql.describe_columns2(dbms_cur, col_cnt, rec_tab);
-- 'print' the description
for j in 1..col_cnt
loop
dbms_output.put_line('Column: '||to_char(j)||' : '||
case rec_tab(j).col_type
when 1 then 'VARCHAR2' -- or NVARCHAR2
when 2 then 'NUMBER'
when 12 then 'DATE'
when 96 then 'CHAR' -- or NCHAR
else 'OTHER DATATYPE'
end||
case when rec_tab(j).col_type in (1,96) then '('||rec_tab(j).col_max_len||')' else null end||' : '||
rec_tab(j).col_name
);
end loop;
-- finally, ensure we close the cursor to release resources
dbms_sql.close_cursor(dbms_cur);
end;
/
Note: the data types are internal data type numbers (see http://docs.oracle.com/cd/E11882_01/appdev.112/e10646/oci03typ.htm#LNOCI16268)
To keep the example simple, I've just included the most common datatypes, but it's easy enough to add more as required.
In the background, I've updated my get_dept_emps function to also include the hiredate from the emp table, so we have a DATE datatype to see (you should be able to figure that out without me showing it)
In SQL*Plus, declare a ref cursor variable (like we did at the very beginning) and obtain our ref cursor...
SQL> var rc refcursor
SQL> exec :rc := get_dept_emps(10);
PL/SQL procedure successfully completed.
Now call our new procedure passing it the ref cursor...
SQL> exec process_ref_cur(:rc);
Column: 1 : NUMBER : EMPNO
Column: 2 : VARCHAR2(10) : ENAME
Column: 3 : NUMBER : MGR
Column: 4 : NUMBER : SAL
Column: 5 : DATE : HIREDATE
PL/SQL procedure successfully completed.
Well look at that! We've managed to get details of the columns in our ref cursor.
How about fetching the data too? No problem...
create or replace procedure process_ref_cur(p_rc in out sys_refcursor) as
dbms_cur integer;
rec_tab dbms_sql.desc_tab2;
col_cnt integer;
-- local variables for fetching data into
fvarchar varchar2(4000);
fnumber number;
fdate date;
--
ret number;
begin
-- Convert the incoming Ref Cursor to a DBMS_SQL cursor
dbms_cur := dbms_sql.to_cursor_number(p_rc);
-- Get the description of the cursor projection.
-- This tells us the number of columns and a structured
-- datatype containing information about each column.
dbms_sql.describe_columns2(dbms_cur, col_cnt, rec_tab);
-- 'print' the description
for j in 1..col_cnt
loop
dbms_output.put_line('Column: '||to_char(j)||' : '||
case rec_tab(j).col_type
when 1 then 'VARCHAR2' -- or NVARCHAR2
when 2 then 'NUMBER'
when 12 then 'DATE'
when 96 then 'CHAR' -- or NCHAR
else 'OTHER DATATYPE'
end||
case when rec_tab(j).col_type in (1,96) then '('||rec_tab(j).col_max_len||')' else null end||' : '||
rec_tab(j).col_name
);
end loop;
-- Fetching the data
-- Define each of the columns
-- This may seem pointless as DBMS_SQL already knows the datatypes of each column, but we may, for example
-- want to fetch all columns back into VARCHAR2 datatypes rather than the datatype of the column
for j in 1..col_cnt
loop
-- for each column based on the datatype specify the local variable we will be fetching it into
case rec_tab(j).col_type
when 1 then dbms_sql.define_column(dbms_cur, j, fvarchar, rec_tab(j).col_max_len);
when 2 then dbms_sql.define_column(dbms_cur, j, fnumber);
when 12 then dbms_sql.define_column(dbms_cur, j, fdate);
when 96 then dbms_sql.define_column(dbms_cur, j, fvarchar, rec_tab(j).col_max_len); -- we'll treat char as varchar2
else -- assume any other datatype we will be able to implicitly convert to varchar2
dbms_sql.define_column(dbms_cur, j, fvarchar, 4000);
end case;
end loop;
-- Fetch each row in a loop, until no row is fetched
loop
-- Tell the cursor to fetch the next row, and return the number of rows fetched
ret := DBMS_SQL.FETCH_ROWS(dbms_cur);
-- if no row was fetched, exit our loop as we've reached the end
exit WHEN ret = 0;-- now read the column value into the variable we defined for that column
-- and print out the data
for j in 1..col_cnt
loop
case rec_tab(j).col_type
when 1 then
dbms_sql.column_value(dbms_cur, j, fvarchar);
dbms_output.put_line(fvarchar);
when 2 then
dbms_sql.column_value(dbms_cur, j, fnumber);
dbms_output.put_line(to_char(fnumber));
when 12 then
dbms_sql.column_value(dbms_cur, j, fdate);
dbms_output.put_line(to_char(fdate,'DD/MM/YYYY HH24:MI:SS'));
when 96 then
dbms_sql.column_value(dbms_cur, j, fvarchar);
dbms_output.put_line(fvarchar);
else
dbms_sql.column_value(dbms_cur, j, fvarchar);
dbms_output.put_line(fvarchar);
end case;
end loop;
end loop;
-- finally, ensure we close the cursor to release resources
dbms_sql.close_cursor(dbms_cur);
end;
/
Note: For more technical details on why we need to "Define" columns before we fetch them, see the Oracle Call Interface Programmer's Guide in the documentation (http://docs.oracle.com/cd/B28359_01/appdev.111/b28395/oci05bnd.htm#i421494)
quote:
Query statements return data from the database to your application. When processing a query, you must define an output variable or an array of output variables for each item in the select-list for retrieving data. The define step creates an association that determines where returned results are stored, and in what format.
...
OCI processes the define call locally on the client side. In addition to indicating the location of buffers where results should be stored, the define step determines what data conversions must take place when data is returned to the application.
Ok, so the code is starting to look a little more complicated as we're having to process each column of the data separately. Let's see it in action...
SQL> var rc refcursor
SQL> exec :rc := get_dept_emps(10);
PL/SQL procedure successfully completed.
SQL> exec process_ref_cur(:rc);
Column: 1 : NUMBER : EMPNO
Column: 2 : VARCHAR2(10) : ENAME
Column: 3 : NUMBER : MGR
Column: 4 : NUMBER : SAL
Column: 5 : DATE : HIREDATE
7782
CLARK
7839
2450
09/06/1981 00:00:00
7839
KING
5000
17/11/1981 00:00:00
7934
MILLER
7782
1300
23/01/1982 00:00:00
PL/SQL procedure successfully completed.
Well that works. We now have details of our columns and we're actually fetching the data and basically formatting it, where necessary, depending on it's datatype, for display purposes.
Ok, it's not very pretty output at the minute, but now we know we can get the column information and we're able to fetch the data, let's finally make some changes to make it look good...
(We'll have to try and size the 'columns' of our output based on the datatype and datatype size, but we should be able to do that with a little substr and padding)
create or replace procedure process_ref_cur(p_rc in out sys_refcursor) as
dbms_cur integer;
rec_tab dbms_sql.desc_tab2;
col_cnt integer;
-- local variables for fetching data into
fvarchar varchar2(4000);
fnumber number;
fdate date;
--
ret number;
header varchar2(4000) := '';
rowcount number := 0;
begin
-- Convert the incoming Ref Cursor to a DBMS_SQL cursor
dbms_cur := dbms_sql.to_cursor_number(p_rc);
-- Get the description of the cursor projection.
-- This tells us the number of columns and a structured
-- datatype containing information about each column.dbms_sql.describe_columns2(dbms_cur, col_cnt, rec_tab);
-- 'print' the description as heading titles
-- and at the same time define each column ready for fetching
-- combining the two sections into onedbms_output.new_line; -- start with a blank line
for j in 1..col_cnt
loop
case rec_tab(j).col_type
when 1 then
dbms_sql.define_column(dbms_cur, j, fvarchar, rec_tab(j).col_max_len);
dbms_output.put(rpad(substr(rec_tab(j).col_name, 1, rec_tab(j).col_max_len), rec_tab(j).col_max_len, ' '));
header := header||lpad('-', rec_tab(j).col_max_len, '-');
when 2 then -- assume numbers will all fit in 10 characters and right align (left pad) names
dbms_sql.define_column(dbms_cur, j, fnumber);
dbms_output.put(lpad(substr(rec_tab(j).col_name, 1, 10), 10, ' '));
header := header||lpad('-', 10, '-');
when 12 then -- assume date format of DD/MM/YYYY HH24:MI:SS so need 19 characters
dbms_sql.define_column(dbms_cur, j, fdate);
dbms_output.put(rpad(substr(rec_tab(j).col_name, 1, 19), 19, ' '));
header := header||lpad('-', 19, '-');
when 96 then
dbms_sql.define_column(dbms_cur, j, fvarchar, rec_tab(j).col_max_len); -- we'll treat char as varchar2
header := header||lpad('-', rec_tab(j).col_max_len, '-');
else -- assume any other datatype we will be able to implicitly convert to varchar2 and display as 30 characters
dbms_sql.define_column(dbms_cur, j, fvarchar, 4000);
dbms_output.put(rpad(substr(rec_tab(j).col_name, 1, 30), 30, ' '));
header := header||lpad('-', 30, '-');
end case;
if j < col_cnt then
dbms_output.put(' '); -- put a space between each column
header := header||' ';
end if;
end loop;
dbms_output.new_line;
dbms_output.put_line(header); -- output the header bar under heading titles
-- Fetching the data
-- Fetch each row in a loop, until no row is fetched
loop
-- Tell the cursor to fetch the next row, and return the number of rows fetched
ret := DBMS_SQL.FETCH_ROWS(dbms_cur);
-- if no row was fetched, exit our loop as we've reached the end
exit WHEN ret = 0;
rowcount := rowcount + 1;-- now print out the data for this row
for j in 1..col_cnt
loop
case rec_tab(j).col_type
when 1 then
dbms_sql.column_value(dbms_cur, j, fvarchar);
dbms_output.put(rpad(NVL(substr(fvarchar,1,rec_tab(j).col_max_len),' '),rec_tab(j).col_max_len,' '));
when 2 then
dbms_sql.column_value(dbms_cur, j, fnumber);
-- we'll right justify the numbers for neatness and determine if they fit in our 10 character number limit
dbms_output.put(case when length(to_char(fnumber)) > 10 then '##########' else lpad(NVL(to_char(fnumber),' '),10,' ') end);
when 12 then
dbms_sql.column_value(dbms_cur, j, fdate);
dbms_output.put(to_char(fdate,'DD/MM/YYYY HH24:MI:SS'));
when 96 then
dbms_sql.column_value(dbms_cur, j, fvarchar);
dbms_output.put(rpad(NVL(substr(fvarchar,1,rec_tab(j).col_max_len),' '),rec_tab(j).col_max_len,' '));
else
dbms_sql.column_value(dbms_cur, j, fvarchar);
-- other datatypes we'll handle roughly by truncating them if they're more than 30 characters
dbms_output.put(rpad(NVL(substr(fvarchar,1,30),' '),30,' '));
end case;
if j < col_cnt then
dbms_output.put(' '); -- put a space between each column
end if;
end loop;
dbms_output.new_line;
end loop;
dbms_output.new_line;
dbms_output.put_line(to_char(rowcount)||' rows selected.');
dbms_output.new_line;
-- finally, ensure we close the cursor to release resources
dbms_sql.close_cursor(dbms_cur);
end;
/
And our final result...
SQL> set serverout on format wrapped
SQL> set feedback off
SQL> var rc refcursor
SQL> exec :rc := get_dept_emps(10);
SQL> exec process_ref_cur(:rc);
EMPNO ENAME MGR SAL HIREDATE
---------- ---------- ---------- ---------- -------------------
7782 CLARK 7839 2450 09/06/1981 00:00:00
7839 KING 5000 17/11/1981 00:00:00
7934 MILLER 7782 1300 23/01/1982 00:00:00
3 rows selected.
SQL>
Now it looks more like we've just issued a select statement in SQL*Plus, or used the print command on a ref cursor, yet our procedure has know prior knowledge of what the ref cursor looks like.
So, why doesn't PL/SQL let us do this natively without having to do all this code using the DBMS_SQL package?
In a sense it already does, but just like the 'print' command in SQL*Plus, when we write a static SQL statement in PL/SQL, or use a cursor loop (or cursor for loop), then PL/SQL does all the opening, defining, binding and fetching for us, making it appear that we can reference the data by column name rather than by position. That's the power of the PL language, to seamlessly integrate SQL within it and abstract all the hard work away from us. That's pretty awesome!
This doesn't just apply to PL/SQL, it applies to any query you do. When you write a SELECT statement directly in SQL*Plus, the SQL*Plus interface does all of the cursor processing for us, opening up a cursor, defining the columns, fetching the rows and formatting the results on the display before closing the cursor. Other tools like SQL*Developer, or TOAD, or PL/SQL Developer etc. all do the same thing. Even when you issue an SQL statement in PL/SQL directly, or as part of an explicit cursor loop etc. most of the hard work is just abstracted away and done for us.
So, the next time you're thinking, "I'll use a ref cursor", just ask yourself "Do I really need to do that?".
5. Summary
In summary, the key things to remember are...
REF CURSORS ARE NOT DATA CONTAINERS. ONCE OPENED, THEY ARE SIMPLY A POINTER TO A QUERY THAT HAS YET TO FETCH DATA.
THEY ARE TYPICALLY CONSUMED BY 3rd PARTY INTERFACES THAT ONLY HANDLE REF CURSORS AND ARE NOT REALLY INTENDED FOR USE WITHIN PL/SQL. WHILST YOU CAN USE THEM IN PL/SQL, ALWAYS FIRST ASK YOURSELF IF YOU "SHOULD".
References:
http://docs.oracle.com/cd/B28359_01/appdev.111/b28370/dynamic.htm
http://docs.oracle.com/cd/B28359_01/appdev.111/b28419/d_sql.htm#BABEDAHF
http://docs.oracle.com/cd/E11882_01/appdev.112/e10646/oci03typ.htm#LNOCI16268
http://docs.oracle.com/cd/B28359_01/appdev.111/b28395/oci05bnd.htm#i421494
Комментарии