Analyze a scenario performance? - asp.net

Question

i want to design something like a dynamic form in which admin define each form fields.
i design 3 table: mainform table for shared properties, then formfield tables which have mainformID as a foreign key and define each form fields
e.g:
AutoID | FormID | FieldName
_____________________________
100 | Form1 | weight
101 | Form1 | height
102 | Form1 | color
103 | Form2 | Size
104 | Form2 | Type
....
at leas a formvalues table like bellow:
FormFieldID | Value | UniqueResponseID
___________________________________________
100 | 50px | 200
101 | 60px | 200
102 | Red | 200
100 | 30px | 201
101 | 20px | 201
102 | Black | 201
103 | 20x10 | 201
104 | Y | 201
....
for each form i have to join these 3 tables to catch all fields and values. i wonder if its the only way to design such a scenario? does it decrease sql performance? or is there any fast and better way?

Answer 1

This is a form of EAV, and I'm gonna assume you absolutely have to do it instead of the "static" design.
does it decrease sql performance?
Yes, getting a bunch of rows (under EAV) is always going to be slower than getting just one (under the static design).
or is there any fast and better way?
Not from the logical standpoint, but there are significant optimizations (for query performance at least) that can be done at the physical level. Specifically, you can carefully design your keys to minimize the I/O (by putting related data close together) and even eliminate the JOIN itself.
For example:
This model migrates keys through FOREIGN KEY hierarchy all the way down to the ATTRIBUTE_VALUE table. The resulting natural composite key in ATTRIBUTE_VALUE table enables us to:
Get all attributes1 of a given form by a single index range scan + table heap access on ATTRIBUTE_VALUE table, and without doing any JOINs at all. In addition to that, you can cluster2 it, eliminating the table heap access and leaving you with only the index range scan3.
If you need to only get the data for a specific response, change the order of the fields in the composite key, so the RESPONSE_ID is at the leading edge.
If you need both "by form" and "by response" queries, you'll need both indexes, at which point, I'd recommend secondary index to also cover4 the VALUE field.
For example:
-- Since we haven't used NONCLUSTERED clause, this is a B-tree
-- that covers all fields. Table heap doesn't exist.
CREATE TABLE ATTRIBUTE_VALUE (
FORM_ID INT,
ATTRIBUTE_NAME VARCHAR(50),
RESPONSE_ID INT,
VALUE VARCHAR(50),
PRIMARY KEY (FORM_ID, ATTRIBUTE_NAME, RESPONSE_ID)
-- FOREIGN KEYs omitted for brevity.
);
-- We have included VALUE, so this B-tree covers all fields as well.
CREATE UNIQUE INDEX ATTRIBUTE_VALUE_IE1 ON
ATTRIBUTE_VALUE (RESPONSE_ID, FORM_ID, ATTRIBUTE_NAME)
INCLUDE (VALUE);
1 Or a specific attribute, or a specific response for a specific attribute.
2 MS SQL Server clusters all tables by default, unless you specify NONCLUSTERED clause.
3 Friendliness to clustering and elimination of JOINs are some of the main strengths of natural keys (as opposed to surrogate keys). But they also make tables "fatter" and don't isolate from ON UPDATE CASCADE. I believe pros outweigh cons in this particular case. For more info on natural vs. surrogate keys, look here.
4 Fortunately, MS SQL Server supports including fields in index solely for covering purposes (as opposed to actually searching through the index). This makes the index leaner than a "normal" index on the same fields.

Answer 2

I like Branko's approach, and it is quite similar to metadata models i have created in the past, so this post is by way of extension to his. you may want to add a datatype table, which can work both for native types (int,varchar,bit,datetime etc.) and your own definitions (although i don't see the necessity off the cuff).
thence, Branko's "value" column becomes:
value_tinyint tinyint
value_int int
value_varchar varchar(xx)
etc.
with a datatype_id (probably tinyint) as a foreign key into the "mydatatype" table.
[excuse the lack of pretty ER diagrams like BD's]
mydatatype
datatype_id tinyint
code varchar(16)
description varchar(64) -- for reference purposes
This extension should:
a. save you a good deal of casting when reading or writing your data
b. allow both reads and writes with some easily constructed dynamic SQL
Furthermore (and maybe this is out of scope), you may want to store the order in which these objects are created/saved, as well as conditional display based on button push/checkbox/radio button selection etc.
I won't go into detail here, since i'm not sure you need these things, but if you do i'll check this every so often and respond with stuff.