SET SQL DIALECT 3;
CREATE TABLE VOIPOPENCACDREPOCH (
    EPOCHID         INTEGER NOT NULL,
    EPOCHSTARTDATE  TIMESTAMP NOT NULL,
    EPOCHENDDATE    TIMESTAMP,
    DESCRIPTION     VARCHAR(1024),
    ISCURRENT       COMPUTED BY (IIF(CURRENT_TIMESTAMP between EPOCHSTARTDATE and
coalesce(EPOCHENDDATE, cast('31.12.2099 23:59:59.9999' as timestamp)),1,0))
);

ALTER TABLE VOIPOPENCACDREPOCH ADD CONSTRAINT PK_VOIPOPENCACDREPOCH PRIMARY KEY (EPOCHID);

CREATE INDEX IDX_VOIPOPENCACDREPOCH_CURRENT ON VOIPOPENCACDREPOCH COMPUTED BY (IIF(CURRENT_TIMESTAMP between EPOCHSTARTDATE and
coalesce(EPOCHENDDATE, cast('31.12.2099 23:59:59.9999' as timestamp)),1,0));
CREATE UNIQUE DESCENDING INDEX IDX_VOIPOPENCACDREPOCH_PERIOD ON VOIPOPENCACDREPOCH (EPOCHSTARTDATE, EPOCHENDDATE);

select * from VOIPOPENCACDREPOCH E where e.iscurrent=0
План
PLAN (E INDEX (IDX_VOIPOPENCACDREPOCH_CURRENT))

select *
from VOIPOPENCACDREPOCH E
where e.iscurrent=1
План
PLAN (E INDEX (IDX_VOIPOPENCACDREPOCH_CURRENT))