Why does the optimizer choose a keylookup instead of 2 separate queries? - sql

Question

I have a table that has a primary key/clustered index on an ID column and a nonclustered index on a system date column. If I query all the columns from the table using the system date column (covering index wouldn't make sense here) the execution plan shows a key lookup because for each record it finds it has to go the the ID to get all of the column data.
The weird thing is, if I write 2 queries with a temp table it performs much faster. I can query the system date to get a table of ID's and then use that table to search the ID column. This makes sense because you're no longer doing the slow key lookup for each record.
Why doesn't the optimizer do this for us?
--slow version with key lookup
--id primary key/clustered index
--systemdate nonclustered index
select ID, col1, col2, col3, col4, col5, SystemDate
from MyTable
where SystemDate > '2019-01-01'
--faster version
--id primary key/clustered index
--systemdate nonclustered index
select ID, SystemDate
into #myTempTable
from MyTable
where SystemDate > '2019-01-01'
select t1.ID, t1.col1, t1.col2, t1.col3, t1.col4, t1.col5, t1.SystemDate
from MyTable t1
inner join #myTempTable t2
on t1.ID = t2.ID

Answer 1

Well, in second case you're actually doing a key lookup yourself, aren't you? ; )
Optimizer could perform slower due to outdated (or missing) statistics, fragmented index.
To tell you why it's actually slower, it's best if you'd paste your execution plans here. This would be way easier to explain what happens.

Answer 2

Query optimizer chooses key lookup because the query is not supported by covering index. It has to grab missing columns from table itself:
/*
--slow version with key lookup
--id primary key/clustered index
--systemdate nonclustered index
*/
select ID, col1, col2, col3, col4, col5, SystemDate
from MyTable
where SystemDate > '2019-01-01';
Adding a covering index should boost the performance:
CREATE INDEX my_idx ON MyTable(SystemDate) INCLUDE(col1, col2, col3, col4, col5);
db<>fiddle demo
For query without JOIN:
select ID, col1, col2, col3, col4, col5, SystemDate
from MyTable -- single table
where SystemDate > '2019-01-01';
There is JOIN in execution plan:
After introducing covering index there is no need for additional key lookup: