Sybase ASE - Performance Select Into Vs Insert Select

Question

I have seen that the performance of a select into is better than a insert/select combination as it is un-logged. But today I encountered a different scenario. I am doing a select into from a physical table into a temp table and it inserts 2.4M records in 410 seconds, but when I split this into a select into statement to create the table first and then insert the data using a insert/select the same query clicks 2.4M records in 10.6 seconds.

I am confused, what can be the reason?

Query 1
=======
select a, b, c
into #A
from table1
where a = 'XYZ' 
and c = 'ABC'

2.4M records inserted in 410 seconds

Query 2
=======
select a, b, c
into #A
from table1
where 1=2

insert #A
select a, b, c
from table1
where a = 'XYZ' 
and c = 'ABC'

2.4M records inserted in 10.6 seconds

After comment from Jean

Now its acting strange. If I execute query 1 first and then query 2, then the results are as above. But if I switch the order, query 1 runs in ~9 secs and query 2 in ~7 secs.

Plans

Query 1

-------
""
QUERY PLAN FOR STATEMENT 1 (at line 1).
""
""
STEP 1
The type of query is DECLARE.
""
""
QUERY PLAN FOR STATEMENT 2 (at line 2).
""
""
STEP 1
The type of query is SELECT.
""
1 operator(s) under root
""
|ROOT:EMIT Operator (VA = 1)
|
|   |SCALAR Operator (VA = 0)
""
""
""
QUERY PLAN FOR STATEMENT 3 (at line 3).
""
""
STEP 1
The type of query is CREATE TABLE.
""
STEP 2
The type of query is INSERT.
""
5 operator(s) under root
""
|ROOT:EMIT Operator (VA = 5)
|
|   |INSERT Operator (VA = 4)
|   |  The update mode is direct.
|   |
|   |   |NESTED LOOP JOIN Operator (VA = 3) (Join Type: Inner Join)
|   |   |
|   |   |   |SCAN Operator (VA = 0)
|   |   |   |  FROM OR List
|   |   |   |  OR List has up to 3 rows of OR/IN values.
|   |   |
|   |   |   |RESTRICT Operator (VA = 2)(0)(0)(0)(10)(0)
|   |   |   |
|   |   |   |   |SCAN Operator (VA = 1)
|   |   |   |   |  FROM TABLE
|   |   |   |   |  ABCD.dbo.ITR
|   |   |   |   |  a
|   |   |   |   |  Using Clustered Index.
|   |   |   |   |  Index : ITR_typ_imnt_cl_optn
|   |   |   |   |  Forward Scan.
|   |   |   |   |  Positioning by key.
|   |   |   |   |  Keys are:
|   |   |   |   |    id_typ_imnt ASC
|   |   |   |   |  Using I/O Size 16 Kbytes for data pages.
|   |   |   |   |  With LRU Buffer Replacement Strategy for data pages.
|   |
|   |  TO TABLE
|   |  #A
|   |  Using I/O Size 16 Kbytes for data pages.
""
""
""
QUERY PLAN FOR STATEMENT 4 (at line 9).
""
""
STEP 1
The type of query is SELECT.
""
1 operator(s) under root
""
|ROOT:EMIT Operator (VA = 1)
|
|   |SCALAR Operator (VA = 0)
""
""
Parse and Compile Time 0.
Adaptive Server cpu time: 0 ms.
Adaptive Server cpu time: 0 ms.  Adaptive Server elapsed time: 0 ms.
Table: #A scan count 0, logical reads: (regular=2452707 apf=0 total=2452707), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Table: ABCD.dbo.ITR (a) scan count 3, logical reads: (regular=581980 apf=0 total=581980), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Execution Time 75.
Adaptive Server cpu time: 7500 ms.  Adaptive Server elapsed time: 7546 ms.
Adaptive Server cpu time: 0 ms.  Adaptive Server elapsed time: 0 ms.

Query 2

-------
""
QUERY PLAN FOR STATEMENT 1 (at line 1).
""
""
STEP 1
The type of query is DECLARE.
""
""
QUERY PLAN FOR STATEMENT 2 (at line 2).
""
""
STEP 1
The type of query is SELECT.
""
1 operator(s) under root
""
|ROOT:EMIT Operator (VA = 1)
|
|   |SCALAR Operator (VA = 0)
""
""
""
QUERY PLAN FOR STATEMENT 3 (at line 3).
""
""
STEP 1
The type of query is CREATE TABLE.
""
STEP 2
The type of query is INSERT.
""
3 operator(s) under root
""
|ROOT:EMIT Operator (VA = 3)
|
|   |INSERT Operator (VA = 2)
|   |  The update mode is direct.
|   |
|   |   |RESTRICT Operator (VA = 1)(4)(0)(0)(0)(0)
|   |   |
|   |   |   |SCAN Operator (VA = 0)
|   |   |   |  FROM TABLE
|   |   |   |  CERD_CORPORATE..INSTRUMENT
|   |   |   |  Table Scan.
|   |   |   |  Forward Scan.
|   |   |   |  Positioning at start of table.
|   |   |   |  Using I/O Size 2 Kbytes for data pages.
|   |   |   |  With LRU Buffer Replacement Strategy for data pages.
|   |
|   |  TO TABLE
|   |  #A
|   |  Using I/O Size 16 Kbytes for data pages.
""
""
""
QUERY PLAN FOR STATEMENT 4 (at line 5).
""
""
STEP 1
The type of query is INSERT.
""
5 operator(s) under root
""
|ROOT:EMIT Operator (VA = 5)
|
|   |INSERT Operator (VA = 4)
|   |  The update mode is direct.
|   |
|   |   |NESTED LOOP JOIN Operator (VA = 3) (Join Type: Inner Join)
|   |   |
|   |   |   |SCAN Operator (VA = 0)
|   |   |   |  FROM OR List
|   |   |   |  OR List has up to 3 rows of OR/IN values.
|   |   |
|   |   |   |RESTRICT Operator (VA = 2)(0)(0)(0)(10)(0)
|   |   |   |
|   |   |   |   |SCAN Operator (VA = 1)
|   |   |   |   |  FROM TABLE
|   |   |   |   |  CERD_CORPORATE.dbo.INSTRUMENT
|   |   |   |   |  a
|   |   |   |   |  Using Clustered Index.
|   |   |   |   |  Index : INSTRUMENT_typ_imnt_cl_optn
|   |   |   |   |  Forward Scan.
|   |   |   |   |  Positioning by key.
|   |   |   |   |  Keys are:
|   |   |   |   |    id_typ_imnt ASC
|   |   |   |   |  Using I/O Size 16 Kbytes for data pages.
|   |   |   |   |  With LRU Buffer Replacement Strategy for data pages.
|   |
|   |  TO TABLE
|   |  #A
|   |  Using I/O Size 2 Kbytes for data pages.
""
""
""
QUERY PLAN FOR STATEMENT 5 (at line 11).
""
""
STEP 1
The type of query is SELECT.
""
1 operator(s) under root
""
|ROOT:EMIT Operator (VA = 1)
|
|   |SCALAR Operator (VA = 0)
""
""
Parse and Compile Time 0.
Adaptive Server cpu time: 0 ms.
Adaptive Server cpu time: 0 ms.  Adaptive Server elapsed time: 0 ms.
Table: #A scan count 0, logical reads: (regular=1 apf=0 total=1), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Table: CERD_CORPORATE..INSTRUMENT scan count 0, logical reads: (regular=0 apf=0 total=0), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Adaptive Server cpu time: 0 ms.  Adaptive Server elapsed time: 0 ms.
Table: #A scan count 0, logical reads: (regular=2493221 apf=0 total=2493221), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Table: CERD_CORPORATE.dbo.INSTRUMENT (a) scan count 3, logical reads: (regular=581980 apf=0 total=581980), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Execution Time 96.
Adaptive Server cpu time: 9600 ms.  Adaptive Server elapsed time: 9580 ms.
Adaptive Server cpu time: 0 ms.  Adaptive Server elapsed time: 0 ms.

Answer 1

Possibly the data is in cache, the first time takes some time, the second time it does a similar thing it goes faster because it doesn't have any physical io.