Optimization of Sybase 15.5 union query - tsql

Question

im having trouble trying to optimize the following query on Sybase 15.5. Does anyone know how could i improve it. Each one of the tables used there have about 30 million rows each. I tried my best to optimize it but still taking lot of time(1.5 hours).
create table #tmp1( f_id smallint, a_date smalldatetime )
create table #tmp2( f_id smallint, a_date smalldatetime )
insert #tmp1
select f_id, a_date = max( a_date )
FROM audit_table
WHERE i_date = #pIDate
group by f_id
insert #tmp2
select f_id , a_date = max( a_date )
FROM n_audit_table
WHERE i_date = #pIDate
group by f_id
create table #tmp(
t_account varchar(32) not null,
t_id varchar(32) not null,
product varchar(64) null
)
insert into #tmp
select t_account,t_id, product
FROM audit_table nt, #tmp1 a
WHERE i_date = #pIDate
and nt.a_date = a.a_date
and nt.f_id = a.f_id
union
select t_account,t_id, product
FROM n_audit_table t, #tmp2 a
WHERE t.item_date = #pIDate
and t.a_date = a.a_date
and t.f_id = a.f_id
Both the tables having indexes on i_date, a_date, f_id. Please find below showplan where it is long time.
QUERY PLAN FOR STATEMENT 2 (at line 24).
Optimized using Serial Mode
STEP 1
The type of query is INSERT.
10 operator(s) under root
|ROOT:EMIT Operator (VA = 10)
|
| |INSERT Operator (VA = 9)
| | The update mode is direct.
| |
| | |HASH UNION Operator (VA = 8) has 2 children.
| | | Using Worktable1 for internal storage.
| | | Key Count: 3
| | |
| | | |NESTED LOOP JOIN Operator (VA = 3) (Join Type: Inner Join)
| | | |
| | | | |SCAN Operator (VA = 0)
| | | | | FROM TABLE
| | | | | #tmp1
| | | | | a
| | | | | 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.
| | | |
| | | | |RESTRICT Operator (VA = 2)(5)(0)(0)(0)(0)
| | | | |
| | | | | |SCAN Operator (VA = 1)
| | | | | | FROM TABLE
| | | | | | audit_table
| | | | | | nt
| | | | | | Index : IX_audit_table
| | | | | | Forward Scan.
| | | | | | Positioning by key.
| | | | | | Keys are:
| | | | | | i_date ASC
| | | | | | a_date ASC
| | | | | | Using I/O Size 2 Kbytes for index leaf pages.
| | | | | | With LRU Buffer Replacement Strategy for index leaf pages.
| | | | | | Using I/O Size 2 Kbytes for data pages.
| | | | | | With LRU Buffer Replacement Strategy for data pages.
| | |
| | | |NESTED LOOP JOIN Operator (VA = 7) (Join Type: Inner Join)
| | | |
| | | | |SCAN Operator (VA = 4)
| | | | | FROM TABLE
| | | | | #tmp2
| | | | | a
| | | | | 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.
| | | |
| | | | |RESTRICT Operator (VA = 6)(5)(0)(0)(0)(0)
| | | | |
| | | | | |SCAN Operator (VA = 5)
| | | | | | FROM TABLE
| | | | | | n_audit_table
| | | | | | t
| | | | | | Index : IX_n_audit_table
| | | | | | Forward Scan.
| | | | | | Positioning by key.
| | | | | | Keys are:
| | | | | | i_date ASC
| | | | | | a_date ASC
| | | | | | Using I/O Size 2 Kbytes for index leaf pages.
| | | | | | With LRU Buffer Replacement Strategy for index leaf pages.
| | | | | | Using I/O Size 2 Kbytes for data pages.
| | | | | | With LRU Buffer Replacement Strategy for data pages.
| |
| | TO TABLE
| | #tmp
| | Using I/O Size 2 Kbytes for data pages.
Total estimated I/O cost for statement 2 (at line 24): 29322945.

Answer 1

I doubt its a union issue. Queries are more probable troublemaker.
I suppose you should start from adding indexes on your temp tables:
create table #tmp1( f_id smallint, a_date smalldatetime )
Create clustered index IX1Temp on #tmp1(f_id )
Create clustered index IX2Temp on #tmp1(a_date )
...
Also, I see not much sense in #tmp1, #tmp2 the way you use them. You could call CTE instead. Also. I would recommend you to try PARTITION BY instead GROUP BY statement.

Answer 2

According to the query execution plan, the problem is the table scans on the temporary tables.
Please get the execution plan for the following query:
insert into #tmp
select t_account,t_id, product
FROM
audit_table nt,
(
select f_id, a_date = max(a_date)
FROM audit_table
WHERE i_date = #pIDate
group by f_id
) a
WHERE
i_date = #pIDate
and nt.a_date = a.a_date
and nt.f_id = a.f_id
union
select t_account,t_id, product
FROM
n_audit_table t,
(
select f_id , a_date = max( a_date )
FROM n_audit_table
WHERE i_date = #pIDate
group by f_id
) a
WHERE
t.item_date = #pIDate
and t.a_date = a.a_date
and t.f_id = a.f_id

Answer 3

How many rows end up in each of the temporary tables?
Looks like the temporary tables could be replaced by using HAVING, I would need to test it, it is always complicated when your group by is on a single column and you require more columns in the output.
Try running this statement with SET STATISTICS PLANCOST ON and SET STATISTICS IO ON as that would give a good idea of the number of pages that are scanned and if Sybase is going wrong somewhere while optimising the query.