Question

We have the following table:

CREATE TABLE [dbo].[CampaignCustomer](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [CampaignID] [int] NOT NULL,
    [CustomerID] [int] NULL,
    [CouponCode] [nvarchar](20) NOT NULL,
    [CreatedDate] [datetime] NOT NULL,
    [ModifiedDate] [datetime] NULL,
    [Active] [bit] NOT NULL,
 CONSTRAINT [PK_CampaignCustomer] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

and the following Unique Index:

CREATE UNIQUE NONCLUSTERED INDEX [IX_CampaignCustomer_CouponCode] ON [dbo].[CampaignCustomer] 
(
    [CouponCode] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON, FILLFACTOR = 20) ON [PRIMARY]
GO

We do pretty constant queries using the CouponCode and other foreign keys (not shown above for simplicity). The CampaignCustomer table has almost 4 million records and growing. We also do campaigns that don’t require Coupon Codes and therefore we don’t insert those records. Now we need to also start tracking those campaigns as well for another purpose. So we have 2 options:

1. We change the CouponCode column ot allow nulls and create a unique filetered index to not include nulls and allow the table to grow even bigger and faster.
2. Create a separate table for tracking all campaigns for this specific purpose.

Keep in mind that the CampaignCustomer table is used very often for redeeming coupons and inserting new ones. Bottom line is we don’t want our customer to redeem a coupon and stay waiting until they give up or for other processes to fail. So, from an efficiency perspective, which option do you think is best and why?