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Reset sequence START WITH

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Two things contributed to this post. One was a student question about the difference between the MAX_VALUE of a sequence and the actual sequence value. The other was a comment on an earlier post addressing an NDS approach to resetting sequences.

The student wanted to understand why there were gaps in the sequence, since they created it with the default values, like this:

CREATE SEQUENCE sequence_name;

A default sequence in an Oracle database sets the cache value to 20, which means you get gaps every time a new set is allocated. This Ask Tom question and answer holds that information.

The blog comment was on an NDS statement post. I wasn’t quite sure what the comment wanted because there really wasn’t a question or a complete code example. Having demonstrated how to dynamically drop and recreate a new sequence without a gap in a comment reply, I thought that was adequate.

Having pointed the student to the Ask Tom column and my answer, he wanted to know how to create a stored procedure to reset sequences dynamically. It took me a couple weeks to get back to this but here’s the procedure. The tricky element of the procedure is the “no binding values allowed” restriction placed on NDS statements that process DDL statements.

The procedure uses two local procedures and two local functinons. The local find_sequence finds a sequence name in the schema, and find_next_sequence function returns the next value. The local create_sequence and drop_sequence procedures respectively isolate the logic for dynamic drops and creates of the sequence.

Warning: The restriction with this design assumes that the table name and sequence name are linked by using the table name and an _ID suffix.

-- Declare an autonomous procedure to drop sequences.
CREATE OR REPLACE PROCEDURE reset_sequence
( pv_table_name     VARCHAR2
, pv_sequence_name  VARCHAR2
, pv_cache_value    NUMBER DEFAULT 0 ) IS
 
  -- Declares stubs to remove forward reference limits.
  PROCEDURE create_sequence ( pv_sequence_name  VARCHAR2, pv_cache_value  NUMBER );
  PROCEDURE drop_sequence ( pv_sequence_name  VARCHAR2 );
  FUNCTION find_sequence ( pv_sequence_name  VARCHAR2 ) RETURN VARCHAR2;
  FUNCTION find_next_sequence ( pv_table_name  VARCHAR2 ) RETURN VARCHAR2;
 
  -- Drops sequence.
  PROCEDURE drop_sequence
  ( pv_sequence_name  VARCHAR2 ) IS
    -- Declare local variable(s).
    lv_statement      VARCHAR2(200);
    lv_sequence_name  VARCHAR2(30);
  BEGIN
 
    /*  Conditionally drop any sequence using a local function to find a valid
        sequence name before attempting to drop it. */  
    IF find_sequence(DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name)) > 0 THEN  
 
      /* Dynamically drop sequence, which requires concatenating the sanitized 
         sequence name because you can't bind values on a DDL statement. */
      lv_statement := 'DROP sequence '||DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name);
 
      -- Execute statement immediately.
      EXECUTE IMMEDIATE lv_statement;
 
    END IF;      
  END drop_sequence;
 
    -- Drops sequence.
  PROCEDURE create_sequence
  ( pv_sequence_name  VARCHAR2
  , pv_cache_value    NUMBER ) IS
    -- Declare local variable(s).
    lv_statement      VARCHAR2(200);
    lv_next_sequence  NUMBER;
  BEGIN
 
    -- Assign the next sequence value if one exists.
    lv_next_sequence := find_next_sequence(pv_table_name);
 
    -- Check whether a cache value has been provided.
    IF pv_cache_value > 0 THEN
 
      /* Dynamically create a sequence statement with a new start value that is one
         greater than the highest value in the table that uses the sequence. */
      lv_statement := 'CREATE SEQUENCE '||DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name)||CHR(10)
                   ||       'MINVALUE 1 NOMAXVALUE INCREMENT BY 1'||CHR(10)
                   ||       'START WITH '||lv_next_sequence||' CACHE '||pv_cache_value||' NOORDER NOCYCLE';
 
    ELSE
 
      /* Dynamically create a sequence statement with a new start value that is one
         greater than the highest value in the table that uses the sequence. */
      lv_statement := 'CREATE SEQUENCE '||DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name)||CHR(10)
                   ||       ' MINVALUE 1 NOMAXVALUE'||CHR(10)
                   ||       'INCREMENT BY 1 START WITH '||lv_next_sequence||' NOCACHE NOORDER NOCYCLE';
 
    END IF;
 
    -- Execute create sequence statement.
    EXECUTE IMMEDIATE lv_statement;
 
  END create_sequence;
 
  -- Finds whether a sequence exists.
  FUNCTION find_sequence
  ( pv_sequence_name  VARCHAR2 ) RETURN VARCHAR2 IS
    -- Declare local return variable(s).
    lv_statement      VARCHAR2(200);
    lv_sequence_name  VARCHAR2(30);
    lv_return_value   NUMBER := 0;
 
    -- Declare local system reference cursor.
    lv_cursor  SYS_REFCURSOR;
 
  BEGIN
    -- Dynamically find sequence.
    lv_statement := 'SELECT   sequence_name'||CHR(10)
                 || 'FROM     user_sequences'||CHR(10)
                 || 'WHERE    sequence_name = :seq_name';
 
    OPEN lv_cursor FOR lv_statement USING DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name);
    FETCH lv_cursor INTO lv_sequence_name;
    CLOSE lv_cursor;
 
    -- Convert valid sequence name to positive integer, which represents truth.
    lv_return_value := LENGTH(lv_sequence_name);
 
    -- Return value.
    RETURN lv_return_value;
  EXCEPTION
    -- Reached when DBMS_ASSERT check fails.
    WHEN OTHERS THEN
      RETURN lv_return_value;
  END find_sequence;
 
  -- Finds the next sequence value.
  FUNCTION find_next_sequence
  ( pv_table_name  VARCHAR2 ) RETURN VARCHAR2 IS
    -- Declare local return variable(s).
    lv_statement      VARCHAR2(200);
    lv_sequence_value  NUMBER;
    lv_return_value    NUMBER := 1;
 
    -- Declare local system reference cursor.
    lv_cursor  SYS_REFCURSOR;
 
  BEGIN
    -- Dynamically find sequence.
    lv_statement := 'SELECT   MAX('||DBMS_ASSERT.SIMPLE_SQL_NAME(pv_table_name)||'_ID) + 1'||CHR(10)
                 || 'FROM    '||DBMS_ASSERT.SIMPLE_SQL_NAME(pv_table_name);
 
    OPEN lv_cursor FOR lv_statement;
    FETCH lv_cursor INTO lv_sequence_value;
    CLOSE lv_cursor;
 
    -- Assign the return value from the NDS statement to a local return variable.
    lv_return_value := lv_sequence_value;
 
    -- Return value.
    RETURN lv_return_value;
  EXCEPTION
    -- Reached when DBMS_ASSERT check fails.
    WHEN OTHERS THEN
      RETURN lv_return_value;
  END find_next_sequence;
 
  -- Set precompiler instruction to run in a discrete process.
  PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
 
  -- Conditionally drop sequence.
  drop_sequence(DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name));
 
  -- Create sequence.
  create_sequence(DBMS_ASSERT.SIMPLE_SQL_NAME(pv_sequence_name), pv_cache_value);
 
END;
/

You can test this procedure with the following script, which includes COMMIT statements. Those are requires because the stand alone procedure runs independently of the current session as an autonomous procedure.

-- Conditionally drop existing tables and sequences.
BEGIN
  FOR i IN (SELECT object_name, object_type FROM user_objects WHERE object_name IN ('SAMPLE','SAMPLE_S')) LOOP
    IF i.object_type = 'TABLE' THEN
      EXECUTE IMMEDIATE 'DROP TABLE '||i.object_name||' CASCADE CONSTRAINTS';
      dbms_output.put_line(i.object_name);
    ELSIF i.object_type = 'SEQUENCE' THEN
      EXECUTE IMMEDIATE 'DROP SEQUENCE '||i.object_name;   
      dbms_output.put_line(i.object_name);
    END IF;
  END LOOP;
END;
/
 
-- Create test table.
CREATE TABLE sample
(sample_id  NUMBER);
 
-- Create test table.
CREATE SEQUENCE sample_s;
 
-- Insert a seeding row.
INSERT INTO sample VALUES (10);
 
-- Commit the values because the procedure is autonomous.
COMMIT;
 
-- Reset the sequence value.
EXECUTE reset_sequence('SAMPLE','SAMPLE_S');
 
-- Insert the next sequence value.
INSERT INTO sample VALUES (sample_s.NEXTVAL);
 
-- Commit the values because the procedure is autonomous.
COMMIT;
 
-- Query the result to ensure the numbers are sequential.
SELECT * FROM sample;
 
EXECUTE reset_sequence('SAMPLE','SAMPLE_S',500);
 
-- Insert the next sequence value.
INSERT INTO sample VALUES (sample_s.NEXTVAL);
 
-- Query the result to ensure the numbers are sequential.
SELECT * FROM sample;

Hope this helps anybody interested in automating the process of resetting a START WITH value of an Oracle sequence.

Written by maclochlainn

November 22nd, 2012 at 2:03 pm

3 Responses to 'Reset sequence START WITH'

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  1. [...] MacLochlainns is blogging about reset sequence start with. [...]

  2. Hi,

    Thats a nice info. I have faced this sequences issue when refreshing production data in the dev database. The sequences which have been imported do not carry the same values as in production. How can we address this issue when we are refreshing the whole database which contains more than 100 sequences?

    RJ

    27 Nov 12 at 1:12 pm

  3. You can put the table and sequence names in a table an run an anonymous block like this:

    BEGIN
      FOR i IN (SELECT name_of_table, name_of_sequence FROM table_list) LOOP
        reset_sequence(i.name_of_table, i.name_of_sequence);
      END LOOP;
    END;
    /

    Unfortunately, prior to Oracle 12c sequences aren’t associated with tables.

    maclochlainn

    27 Nov 12 at 5:40 pm

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