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Therefore, PL/SQL was developed to utilize an asynchronous commit, allowing the COMMIT statement in PL/SQL to not have to wait for the physical I/O to complete (avoiding the log file sync wait). That does not mean that you can t rely on a PL/SQL routine that commits and returns control to your application to not be durable with respect to its changes PL/SQL will wait for the redo it generated to be written to disk before returning to the client application but it will only wait once, right before it returns.

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Note The following example demonstrates a bad practice one that I call slow-by-slow processing or rowby-row processing, as row-by-row is synonymous with slow-by-slow in a relational database. It is meant just to illustrate how PL/SQL processes a commit statement.

For example: seq { for i in numbers do if i % 2 = 0 then yield (i, i*i) } |> Seqtruncate 3 |> sortBy revOrder There are pros and cons to using sequence expression syntax for some parts of queries: Sequence expressions are very good for the subset of queries expressed using iteration (for), mapping (select/yield), and filtering (if/then/when/where) They are particularly good for queries containing multiple nested for statements Other query constructs such as ordering, truncating, grouping, and aggregating must be expressed directly using aggregate operators such as SeqorderBy and SeqgroupBy Some queries depend on the index position of an item within a stream These are best expressed directly using aggregate operators such as Seqmapi Many queries are often part of a longer series of transformations chained by |> operators.

Consider this PL/SQL procedure: ops$tkyte%ORA11GR2> create table t 2 as 3 select * 4 from all_objects 5 where 1=0 6 / Table created. ops$tkyte%ORA11GR2> create or replace procedure p 2 as 3 begin 4 for x in ( select * from all_objects ) 5 loop 6 insert into t values X; 7 commit; 8 end loop; 9 end; 10 / Procedure created. That PL/SQL code reads a record at a time from ALL_OBJECTS, inserts the record into table T and commits each record as it is inserted. Logically, that code is the same as: ops$tkyte%ORA11GR2> create or replace procedure p 2 as 3 begin 4 for x in ( select * from all_objects ) 5 loop 6 insert into t values X; 7 commit write NOWAIT; 8 end loop; 9 10 -- make internal call here to ensure 11 -- redo was written by LGWR 12 end; 13 / Procedure created. So, the commits performed in the routine are done with WRITE NOWAIT and before the PL/SQL block of code returns to the client application, PL/SQL makes sure that the last bit of redo it generated was safely recorded to disk making the PL/SQL block of code and its changes durable.

Table created. ops$tkyte%ORA11GR2> create index 2 t_idx on 3 t( decode( processed_flag, 'N', 'N' ) ); Index created. ops$tkyte%ORA11GR2> ops$tkyte%ORA11GR2> insert into t 2 select r, 3 case when mod(r,2) = 0 then 'N' else 'Y' end, 4 'payload ' || r 5 from (select level r 6 from dual 7 connect by level <= 5) 8 / 5 rows created. ops$tkyte%ORA11GR2> select * from t; ID ---------1 2 3 4 5 P Y N Y N Y PAYLOAD -------------------payload 1 payload 2 payload 3 payload 4 payload 5

Often the type of the data being transformed at each step varies substantially through the chain of operators These queries are best expressed using aggregate operator chains..

Then we basically need to find any and all unprocessed records. One by one we ask the database Is this row locked already If not, then lock it and give it to me. That code would look like: ops$tkyte%ORA11GR2> create or replace 2 function get_first_unlocked_row 3 return t%rowtype 4 as 5 resource_busy exception; 6 pragma exception_init( resource_busy, -54 ); 7 l_rec t%rowtype; 8 begin 9 for x in ( select rowid rid 10 from t 11 where decode(processed_flag,'N','N') = 'N') 12 loop 13 begin 14 select * into l_rec 15 from t 16 where rowid = x.rid and processed_flag='N' 17 for update nowait; 18 return l_rec; 19 exception 20 when resource_busy then null; when no_data_found then null;

OR REPLACE. Right before DDL runs, it automatically commits, so there was an implicit COMMIT in there. The rows we ve inserted are committed in the database and that fact is necessary for the following examples to work correctly. In general, I ll use that fact in the remainder of the book. If you run these examples without performing the CREATE OR REPLACE, make sure to COMMIT first!