Many to many relationship with a reverse lookup - mysql

Question

MySQL version 5.5.35-log
I have an exceptionally large set of data which consists of a many-to-many relationship which is closely related to people shopping at outlets. A person may shop at many hundreds of different outlets, and similarly, many thousands of people may shop at any particular outlet. The overall number of people and outlets extends into the millions each.
I have a situation where checking if a person shops at a particular outlet must be resolved quickly, so I opted to use reverse lookups; i.e. each 'person' row stores a list of ID's for the stores they shop in. Due to the volume of data, a third relationship table is presumed to be unsuitable; i.e. one which has a row for each persons outlets. My assumption here is that it would have little choice but to produce table scans through many, many rows.
To store this reverse lookup in MySQL however, SET is also unsuitable as it has a maximum of 64 entries which is of course not enough in this situation. So, I opted for a BLOB which is structured as simply a block containing each 4 byte little-endian ID.
But, a different problem arises here; When it comes time to find if an outlet ID is contained in the BLOB using SQL, unusual things start occuring. From other questions, it seems the only way to do this is using SUBSTRING with the BLOB in a loop, however this doesn't seem to work; SUBSTRING is returning a blank string. First, here's some code:
CREATE FUNCTION `DoesShopAt`(shopperID INT UNSIGNED,outletID TINYBLOB) RETURNS VARCHAR(20)
BEGIN
-- Setup a loop. We're going to loop through each ID in the blob:
declare i_max int unsigned default 0;
declare i int unsigned default 0;
declare offset int unsigned default 0;
declare storeID tinyblob;
-- Setup the blob store - all the stops a particular shopper goes to:
declare allShops blob;
-- Grab the set of ID's - a blob of each 4 byte outlet ID:
select AllStores from Shoppers where ID=shopperID into allShops;
-- How many shops?
select length(allShops)/4 into i_max;
while i < i_max do
-- Grab the shops ID:
set storeID=substring(allShops,offset,4);
if outletID = storeID then
return "Yep, they do!";
end if;
-- Update the ID offset in the blob:
set offset=offset+4;
-- Update the loop counter:
set i=i+1;
end while;
return "Nope, they don't.";
END
For debugging purposes it is set to return a string. The intention is it returns true or false depending on if the given shopper does shop at the given outlet.
Ideally, this function would receive two numbers; the shopperID and the outletID, however converting the outletID into a block of 4 little endian bytes seems unreliable and slow at best as it must go via hex (as far as I can tell). So instead, the calling service provides the block of 4 bytes.
Interestingly though, returning storeID immediately after it is set results in a blank string. This is the case if the type of storeID is a varchar, binary or tinyblob; it seems no matter what, it is returning a blank string.
So as a final resort for testing purposes, I instead tried this:
set storeID=substring(hex(allShops),offset,8);
Ensuring that the offset counter was increased by 8 this time, and the input ID was adjusted to suit. Yet again though, it still was returning a blank string (again with return storeID immediately after it's set), even though the allShops data is non-zero.
Edit: Although I found the issue, I can't help but think that maybe there is a better approach to reverse lookups like this in MySQL; do you have any suggestions?

Answer 1

I started playing around with substring and realised what the issue was; offset is being initialised to 0 when it should be 1. Changing this then started correctly returning results:
declare offset int unsigned default 0;
Should have been:
declare offset int unsigned default 1;
However, please see the note at the bottom of the original question.