<!--
-$PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.55 2005/01/08 22:13:34 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.56 2005/01/14 01:16:22 tgl Exp $
-->
<chapter id="plpgsql">
FETCH ALL IN "<unnamed cursor 1>";
COMMIT;
+</programlisting>
+ </para>
+
+ <para>
+ The following example shows one way to return multiple cursors
+ from a single function:
+
+<programlisting>
+CREATE FUNCTION myfunc(refcursor, refcursor) RETURNS SETOF refcursor AS $$
+BEGIN
+ OPEN $1 FOR SELECT * FROM table_1;
+ RETURN NEXT $1;
+ OPEN $2 FOR SELECT * FROM table_2;
+ RETURN NEXT $2;
+ RETURN;
+END;
+$$ LANGUAGE plpgsql;
+
+-- need to be in a transaction to use cursors.
+BEGIN;
+
+SELECT * FROM myfunc('a', 'b');
+
+FETCH ALL FROM a;
+FETCH ALL FROM b;
+COMMIT;
</programlisting>
</para>
</sect3>
CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
FOR EACH ROW EXECUTE PROCEDURE emp_stamp();
</programlisting>
-
-
</example>
<para>
FOR EACH ROW EXECUTE PROCEDURE process_emp_audit();
</programlisting>
</example>
+
+ <para>
+ One use of triggers is to maintain a summary table
+ of another table. The resulting summary can be used in place of the
+ original table for certain queries — often with vastly reduced run
+ times.
+ This technique is commonly used in Data Warehousing, where the tables
+ of measured or observed data (called fact tables) can be extremely large.
+ <xref linkend="plpgsql-trigger-summary-example"> shows an example of a
+ trigger procedure in <application>PL/pgSQL</application> that maintains
+ a summary table for a fact table in a data warehouse.
+ </para>
+
+
+ <example id="plpgsql-trigger-summary-example">
+ <title>A <application>PL/pgSQL</application> Trigger Procedure For Maintaining A Summary Table</title>
+
+ <para>
+ The schema detailed here is partly based on the <emphasis>Grocery Store
+ </emphasis> example from <emphasis>The Data Warehouse Toolkit</emphasis>
+ by Ralph Kimball.
+ </para>
+
+<programlisting>
+--
+-- Main tables - time dimension and sales fact.
+--
+CREATE TABLE time_dimension (
+ time_key integer NOT NULL,
+ day_of_week integer NOT NULL,
+ day_of_month integer NOT NULL,
+ month integer NOT NULL,
+ quarter integer NOT NULL,
+ year integer NOT NULL
+);
+CREATE UNIQUE INDEX time_dimension_key ON time_dimension(time_key);
+
+CREATE TABLE sales_fact (
+ time_key integer NOT NULL,
+ product_key integer NOT NULL,
+ store_key integer NOT NULL,
+ amount_sold numeric(12,2) NOT NULL,
+ units_sold integer NOT NULL,
+ amount_cost numeric(12,2) NOT NULL
+);
+CREATE INDEX sales_fact_time ON sales_fact(time_key);
+
+--
+-- Summary table - sales by time.
+--
+CREATE TABLE sales_summary_bytime (
+ time_key integer NOT NULL,
+ amount_sold numeric(15,2) NOT NULL,
+ units_sold numeric(12) NOT NULL,
+ amount_cost numeric(15,2) NOT NULL
+);
+CREATE UNIQUE INDEX sales_summary_bytime_key ON sales_summary_bytime(time_key);
+
+--
+-- Function and trigger to amend summarized column(s) on UPDATE, INSERT, DELETE.
+--
+CREATE OR REPLACE FUNCTION maint_sales_summary_bytime() RETURNS TRIGGER AS $maint_sales_summary_bytime$
+ DECLARE
+ delta_time_key integer;
+ delta_amount_sold numeric(15,2);
+ delta_units_sold numeric(12);
+ delta_amount_cost numeric(15,2);
+ BEGIN
+
+ -- Work out the increment/decrement amount(s).
+ IF (TG_OP = 'DELETE') THEN
+
+ delta_time_key = OLD.time_key;
+ delta_amount_sold = -1 * OLD.amount_sold;
+ delta_units_sold = -1 * OLD.units_sold;
+ delta_amount_cost = -1 * OLD.amount_cost;
+
+ ELSIF (TG_OP = 'UPDATE') THEN
+
+ -- forbid updates that change the time_key -
+ -- (probably not too onerous, as DELETE + INSERT is how most
+ -- changes will be made).
+ IF ( OLD.time_key != NEW.time_key) THEN
+ RAISE EXCEPTION 'Update of time_key : % -> % not allowed', OLD.time_key, NEW.time_key;
+ END IF;
+
+ delta_time_key = OLD.time_key;
+ delta_amount_sold = NEW.amount_sold - OLD.amount_sold;
+ delta_units_sold = NEW.units_sold - OLD.units_sold;
+ delta_amount_cost = NEW.amount_cost - OLD.amount_cost;
+
+ ELSIF (TG_OP = 'INSERT') THEN
+
+ delta_time_key = NEW.time_key;
+ delta_amount_sold = NEW.amount_sold;
+ delta_units_sold = NEW.units_sold;
+ delta_amount_cost = NEW.amount_cost;
+
+ END IF;
+
+
+ -- Update the summary row with the new values.
+ UPDATE sales_summary_bytime
+ SET amount_sold = amount_sold + delta_amount_sold,
+ units_sold = units_sold + delta_units_sold,
+ amount_cost = amount_cost + delta_amount_cost
+ WHERE time_key = delta_time_key;
+
+
+ -- There might have been no row with this time_key (e.g new data!).
+ IF (NOT FOUND) THEN
+ BEGIN
+ INSERT INTO sales_summary_bytime (
+ time_key,
+ amount_sold,
+ units_sold,
+ amount_cost)
+ SELECT f.time_key,
+ sum(f.amount_sold),
+ sum(f.units_sold),
+ sum(f.amount_cost)
+ FROM sales_fact f
+ WHERE f.time_key = delta_time_key
+ GROUP BY f.time_key;
+ -- This query can potentially be very expensive if the trigger
+ -- is created on sales_fact without the time_key indexes.
+ -- Some care is needed to ensure that this situation does
+ -- *not* occur.
+ EXCEPTION
+ --
+ -- Catch race condition when two transactions are adding data
+ -- for a new time_key.
+ --
+ WHEN UNIQUE_VIOLATION THEN
+ UPDATE sales_summary_bytime
+ SET amount_sold = amount_sold + delta_amount_sold,
+ units_sold = units_sold + delta_units_sold,
+ amount_cost = amount_cost + delta_amount_cost
+ WHERE time_key = delta_time_key;
+
+ END;
+ END IF;
+ RETURN NULL;
+
+ END;
+$maint_sales_summary_bytime$ LANGUAGE plpgsql;
+
+CREATE TRIGGER maint_sales_summary_bytime
+AFTER INSERT OR UPDATE OR DELETE ON sales_fact
+ FOR EACH ROW EXECUTE PROCEDURE maint_sales_summary_bytime();
+</programlisting>
+ </example>
+
</sect1>
<!-- **** Porting from Oracle PL/SQL **** -->
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