How to speed up querying same table many times with different conditions? - sql

Question

EF6, asp mvc core and SQL Server are used on the background.
I have to do many queries to the same table with different conditions, f.e.
SELECT COUNT(*) FROM Table1 WHERE a = true
SELECT COUNT(*) FROM Table1 WHERE b = true
SELECT COUNT(*) FROM Table1 WHERE a = true || b = true
SELECT a FROM Table1 WHERE b = true
So 4 queries to Table1 with different conditions. I think that as result I have to read the entire Table1 four times. In pseudo code it might be looking like this.
var res1 = new list();
foreach(var rec in Table1)
{
// read Table1 first time
if(rec.a == true)
{
res1.push(rec);
}
}
var res2 = new list();
foreach(var rec in Table1)
{
// read Table1 second time
if(rec.b == true)
{
res2.push(rec);
}
}
var res3 = new list();
foreach(var rec in Table1)
{
// read Table1 third time
if(rec.a == true || rec.b == true)
{
res3.push(rec);
}
}
var res4 = new list();
foreach(var rec in Table1)
{
// read Table1 fourth time
if(rec.b == true)
{
res4.push(rec);
}
}
I want to know how to read the Table1 only one time and get four different results, like this:
var res1 = new List();
var res2 = new List();
var res3 = new List();
var res4 = new list();
foreach(rec in Table1)
{
// read Table1 first time
if(a == true)
{
res1.push(rec);
}
if(b == true)
{
res2.push(rec);
}
if(a == true || b == true)
{
res3.push(rec);
}
if(b == true)
{
res4.push(rec);
}
}
Also the challenge, that those queries are dynamic sql, I mean that a = true, b = true, a = true || b = true are stored in database. And queries are running in this way:
string query = "SELECT Count(*) FROM Table1 WHERE" + condition;
var count = ExecuteSql(query);
The sample above is simplified, but in reality all the query is split and stored in database.
PS. Actually I want to speed up the page, which makes 30-40 requests to the server and each request is the query to the same table. I think if I can replace them with one request instead of 40 requests.

Answer 1

You may use conditional aggregation with just a single query:
SELECT
COUNT(CASE WHEN a = true THEN 1 END) AS cnt_a,
COUNT(CASE WHEN b = true THEN 1 END) AS cnt_b,
COUNT(CASE WHEN a = true OR b = true THEN 1 END) AS cnt_a_b
FROM Table1;
This would reduce the number of full table scans from 3 to just 1. Also, it would also potentially reduce the number of round trips to/from the database from 3 to 1.