PostgreSQL – Workaround for Very Slow DELETE

• Rather than "VARCHAR(40000)" why not use "TEXT"?
• HASH index use is discouraged (see docs at http://www.postgresql.org/docs/current/static/indexes-types.html).
• Have you run "ANALYZE" on your tables before running the query?
• Giant IN lists can be performance killers.

How do the following queries perform?

SELECT e.entityId
FROM entities e
INNER JOIN triples t ON (t.object = e.entityId)
LIMIT 10000;

or

SELECT e.entityId
FROM entities e
WHERE EXISTS (SELECT 1 FROM triples t WHERE t.object = e.entityId LIMIT 10000);

Make sure that there are indexes on the constrained columns, since the dbms will do a lookup using those columns in the referencing tables.

You can try using this script, it generates an index creation script for any non-indexed column being referenced by a cascading constraints.

SELECT 'CREATE NONCLUSTERED INDEX IX_'+OBJECT_NAME(fk.parent_object_id)+'_'+c.name+' ON '+OBJECT_NAME(fk.parent_object_id)+'('+c.name+') WITH (ONLINE=ON)' 
        --, OBJECT_NAME(fk.referenced_object_id) AS referenced_tale,  cc.name
FROM 
    sys.foreign_keys fk
    INNER JOIN sys.foreign_key_columns fkc ON fk.object_id = fkc.constraint_object_id
    INNER JOIN sys.columns c ON fkc.parent_object_id = c.object_id AND fkc.parent_column_id = c.column_id
    INNER JOIN sys.columns cc ON fkc.referenced_object_id = cc.object_id AND fkc.referenced_column_id = cc.column_id
WHERE delete_referential_action_desc IN ('CASCADE', 'SET_NULL')
    AND NOT EXISTS
        (
        SELECT  1
        FROM
            SYS.index_columns ic 
            INNER JOIN sys.indexes i ON i.object_id = ic.object_id AND ic.index_id = i.index_id
        WHERE 
            1 = 1
            AND type_DESC IN ('CLUSTERED','NONCLUSTERED')
            AND ic.OBJECT_ID = c.object_id
            AND ic.column_id = c.column_id
            AND ic.is_included_column = 0
        )