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POHow can I lock and return multiple rows from an Oracle function?
 

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  1. POHow can I lock and return multiple rows from an Oracle function?
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    <p>I have been trying to address the issue of how Oracle processes <code>ROWNUM</code> and <code>SELECT ... FOR UPDATE SKIP LOCKED</code> while trying to return several rows that aren't locked. I have tried a number of the solutions from the following: <a href="https://stackoverflow.com/questions/6117254/force-oracle-to-return-top-n-rows-with-skip-locked">Force Oracle to return TOP N rows with SKIP LOCKED</a>, as well as several other examples that look very similar to the ones found on that question. I know Oracle AQ is probably the best solution to this, but we have very little control over the databases and I have met with considerable resistance to the idea.</p> <p>The problem I am running into is trying to get the results back to Java using JDBC. I have tried <code>setFetchSize(20)</code>, but I run into the issue where only the top 20 rows are distributed to the clients. I usually see one processing agent getting all 20 rows or a few processors getting some rows, all of them adding up to 20. This is very much like the behavior one would see with using <code>ROWNUM</code> in conjunction with <code>SELECT ... FOR UPDATE SKIP LOCKED</code>.</p> <p>The most promising solution I have tried is the following function:</p> <pre class="lang-sql prettyprint-override"><code>create type IND_ID as object ( ID varchar2(200) ); create type IND_ID_TABLE as table of IND_ID; create or replace function SELECTIDS return IND_ID_TABLE pipelined is ST_CURSOR SYS_REFCURSOR; ID_REC IND_ID := IND_ID(null); begin open ST_CURSOR for select ID from TABLE /* where clause */ for update SKIP LOCKED; loop fetch ST_CURSOR into ID_REC.ID; exit when ST_CURSOR%rowcount &gt; 20 or ST_CURSOR%notfound; pipe row(ID_REC); end loop; close ST_CURSOR; return; end; </code></pre> <p>However, when I try invoking it like so:</p> <pre class="lang-sql prettyprint-override"><code>select * from table(SELECTIDS) </code></pre> <p>I get an <code>ORA-14551: cannot perform a DML operation inside a query</code> error, which I now understand is an issue with transactions. Removing the locks causes the function to return rows.</p> <p>How can I get multiple rows out of this function into JDBC while preserving the locks?</p>
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    1. COCan you use a procedure with a ref cursor out parameter? Or a function returning a ref cursor, I suppose. Then you'd be calling the proc/function directly rather than trying to use the function in a query. Not sure if it would avoid the issue but might be something to play with.
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    2. CO@AlexPoole Per your suggestion, I was able to figure out how to get at least one row out, changing this to a procedure and using an OUT parameter, and it does maintain the lock. I have also found a few PEBKAC errors and I may have an alternate solution working. Thanks.
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