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POAdding logic to a PostgreSQL stored procedure
 

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  1. POAdding logic to a PostgreSQL stored procedure
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    <p>I'm using PostgreSQL (8.3+) and have defined an enum and a table as follows:</p> <pre><code>CREATE TYPE "viewer_action" AS ENUM ('A', 'B', 'C', 'D', 'E', 'F', 'G', 'H'); CREATE TABLE "preferences" ( "user_id" integer NOT NULL, "item_id" integer NOT NULL, "rating" viewer_action NOT NULL, "time_created" timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, PRIMARY KEY ("user_id","video_id") ); </code></pre> <p>I've also created a stored procedure to upsert new rows into the preferences table, using the example from <a href="http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE" rel="nofollow">http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-UPSERT-EXAMPLE</a>:</p> <pre><code>CREATE OR REPLACE FUNCTION add_preference(u INT, i INT, r viewer_action) RETURNS VOID AS $add_preference$ BEGIN LOOP -- first try to update the key UPDATE preferences SET rating = r WHERE user_id = u AND item_id = i; IF found THEN RETURN; END IF; -- not there, so try to insert the key -- if someone else inserts the same key concurrently, -- we could get a unique-key failure BEGIN INSERT INTO preferences(user_id,item_id,rating) VALUES (u,i,r); RETURN; EXCEPTION WHEN unique_violation THEN -- do nothing, and loop to try the UPDATE again END; END LOOP; END; $add_preference$ LANGUAGE plpgsql; </code></pre> <p>I need to add some additional logic to the upsert to prevent some values from overwriting other values. Specifically:</p> <ul> <li>A can be overwritten by B, which can by overwritten by C, which can be overwritten by D, and so on through F. But B cannot by overwritten by A, nor C overwritten by B, etc.</li> <li>F, G, or H can overwrite any value, regardless if an existing value is lower or higher.</li> </ul> <p>In pseudocode, this might look like:</p> <pre><code>if (rating &gt;= F) { insert; } else if (rating &gt; existing_rating) { insert; } else { return; } </code></pre>
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