How To Improve Performance of Sql Query with Union - sql

Question

Below query takes almost 47-50 seconds on our production server, but when I execute the same on a test server with the same database, it executes in less than a second.
CREATE TABLE [dbo].[Table0]
(
[UserID] [VARCHAR](20) NOT NULL,
[Key] [VARCHAR](600) NOT NULL,
[Type] [VARCHAR](20) NOT NULL,
[status] [VARCHAR](10) NOT NULL,
[RecID] [NUMERIC](18, 0) NOT NULL,
CONSTRAINT [PK_Table0]
PRIMARY KEY CLUSTERED ([UserID] ASC, [Key] ASC, [Type] ASC, [status], [RecID] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Table2]
(
[UserID] [VARCHAR](20) NOT NULL,
[RecID] [NUMERIC](18, 0) NOT NULL,
[Activity] [NUMERIC](2, 0) NOT NULL,
[AwsExist] [NUMERIC](2, 0) NULL,
[View] [NUMERIC](2, 0) NULL,
CONSTRAINT [PK_Table2]
PRIMARY KEY CLUSTERED ([UserID] ASC, [RecID] ASC, [Activity] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Table3]
(
[UserID] [VARCHAR](20) NOT NULL,
[RecId] [NUMERIC](18, 0) NOT NULL,
[Activity] [NUMERIC](2, 0) NOT NULL,
[AwsExist] [NUMERIC](2, 0) NULL,
[RecDate] [DATETIME] NULL,
[View] [NUMERIC](2, 0) NULL,
CONSTRAINT [PK_Table3]
PRIMARY KEY CLUSTERED ([UserID] ASC, [RecId] ASC, [Activity] ASC)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
Table0 contains 10,000 Records
Table1 contains 20,00,000 records
Table2 contains 25,00,000 records
which parameter should I check to resolve the issue? I just want to check if a record exists in one of these table for RecID
Select UserID From Table0 Where RecId = 56445 And Type = 'D'
Union
Select UserID From Table1 Where RecId = 56445 And View = 0
Union
Select UserID From Table2 Where RecId = 56445 And View = 0
I tried with this code:
DECLARE RecID AS INT
SET RecID = 56445
IF NOT EXISTS (SELECT DISTINCT TOP(1) USerID
FROM Table0
WHERE RecId = #RecID
AND Type = 'D')
BEGIN
IF NOT EXISTS (SELECT DISTINCT TOP(1) UserID
FROM Table1
WHERE RecId = #RecID AND View = 0)
BEGIN
SELECT DISTINCT TOP(1) UserID
FROM Table2
WHERE RecId = #RecID AND View = 0
END
END
but it also takes around 18 seconds.

Answer 1

This query should be fast -- assuming not many rows are returned:
Select UserID From Table0 Where RecId = 56445 And Type = 'D'
Union
Select UserID From Table1 Where RecId = 56445 And View = 0
Union
Select UserID From Table2 Where RecId = 56445 And View = 0;
What you need are indexes:
Table0(recid, type, userid)
Table1(recid, view, userid)
Table2(recid, view, userid)

Answer 2

You may have to use query like:
Select UserID
From table0
Where exists (
Select 1
From Table0 t0, Table1 t1, Table2 t2
Where (t0.RecId = 56445 and t0.Type= 'D') OR (t1.RecId = 56445 and t1.View= 0) OR (t2.RecId = 56445 AND t2.View=0))