SELECT

Pg.user_identifier, Pg.session_identifier, COUNT(Pg.user_identifier),
EXISTS(
    SELECT * FROM pageviews
    WHERE pageview_current_url_type='BUYSUCCESS'
    AND user_identifier=Pg.user_identifier
    AND session_identifier=Pg.session_identifier
)

FROM predictions Pd, pageviews Pg
WHERE Pd.prediction_value > 0.9
AND Pd.user_identifier=Pg.user_identifier
GROUP BY Pg.user_identifier, Pg.session_identifier;