Why those queries do not utilize indexes?

Hi everybody!

Here is my queries:

1. explain SELECT * FROM messageinfo WHERE user_id = CAST( 20000 AS BIGINT ) and msgstatus = CAST(
0 AS smallint );

2. explain SELECT * FROM messageinfo WHERE messageinfo.user_id = 20000::int8 and msgstatus =
0::smallint;

In both cases Explain command shows:
1. Sequential search and very high cost if set enable_seqscan to on;
Seq scan on messageinfo ( cost=0.00..24371.30, rows =36802 )

2. Index scan but even bigger cost if set enable_seqscan to off;
Index messagesStatus on messageinfo ( Cost=0.00..27220.72, rows=36802 )

messageinfo table has 200 records which meet this criteria and 662420 in total:

CREATE TABLE messageinfo
(
user_id int8 NOT NULL,
msgstatus int2 NOT NULL DEFAULT (0)::smallint,
receivedtime timestamp NOT NULL DEFAULT now(),

msgread bool DEFAULT false,

CONSTRAINT "$1" FOREIGN KEY (user_id) REFERENCES users (id) ON UPDATE CASCADE ON DELETE CASCADE,
)
WITH OIDS;

CREATE INDEX msgstatus
ON messageinfo
USING btree
(user_id, msgstatus);

CREATE INDEX "messagesStatus"
ON messageinfo
USING btree
(msgstatus);

CREATE INDEX msgread
ON messageinfo
USING btree
(user_id, msgread);

CREATE INDEX "receivedTime"
ON messageinfo
USING btree
(receivedtime);

MY QUESTIONS ARE:

1. Should I afraid of high cost indexes? Or query will still be very efficient?

2. Postgres does not use the index I need. For my data sets its always msgstatus index is
narrowest compare with messagesStatus one. Is any way to enforce to use a particular index?
Whats the logic when Postgres chooses one index compare with the other.

3. I can change db structure to utilize Postgres specifics if you can tell them to me.

4. Also, originally I had messagesStatus index having 2 components ( msgstatus, user_id ).
But query SELECT * FROM messageinfo WHERE msgstatus = 0 did not utilize indexes in this case. It
only worked if both index components are in WHERE part. So I have to remove 2-nd component
user_id from messagesStatus index even I wanted it. Is any way that where clause has only 1-st
component but index is utilized?

Igor Artimenko

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