Sybase 15.7: Aggregating data in a large table - sybase

Question

We have a fairly simple stored procedure that aggregates all the data in a large table (and then puts the results in another table). For a 5M rows table this process takes around 4 minutes - which is perfectly reasonable. But for a 13M rows table this same process takes around 60 minutes.
It looks like we are breaking some kind of a threshold and I struggle to find a simple workaround. Manually rewriting this as several "threads" aggregating small portions of table results in a reasonable run time of 10-15 minutes.
Is there a way to see the actual bottleneck here?
Update: The query plans are of course identical for both queries and they look like this:
|ROOT:EMIT Operator (VA = 3)
|
| |INSERT Operator (VA = 2)
| | The update mode is direct.
| |
| | |HASH VECTOR AGGREGATE Operator (VA = 1)
| | | GROUP BY
| | | Evaluate Grouped SUM OR AVERAGE AGGREGATE.
| | | Evaluate Grouped COUNT AGGREGATE.
| | | Evaluate Grouped SUM OR AVERAGE AGGREGATE.
[---//---]
| | | Evaluate Grouped SUM OR AVERAGE AGGREGATE.
| | | Using Worktable1 for internal storage.
| | | Key Count: 10
| | |
| | | |SCAN Operator (VA = 0)
| | | | FROM TABLE
| | | | TableName
| | | | Table Scan.
| | | | Forward Scan.
| | | | Positioning at start of table.
| | | | Using I/O Size 16 Kbytes for data pages.
| | | | With MRU Buffer Replacement Strategy for data pages.
| |
| | TO TABLE
| | #AggTableName
| | Using I/O Size 2 Kbytes for data pages.
Update 2:
Some statistics:
5M rows 13M rows
CPUTime 263,350 1,180,700
WaitTime 577,574 1,927,399
PhysicalReads 2,304,977 13,704,583
LogicalReads 11,479,123 27,911,085
PagesRead 5,550,737 19,518,030
PhysicalWrites 131,924 5,557,143
PagesWritten 263,640 6,103,708
Update 3:
set statistics io,time on results reformatted for easier comparison:
5M rows 13M rows
+-------------------------+-----------+------------+
| #AggTableName | | |
| logical reads regular | 81 114 | 248 961 |
| apf | 0 | 0 |
| total | 81 114 | 248 961 |
| physical reads regular | 0 | 2 |
| apf | 0 | 0 |
| total | 0 | 2 |
| apf IOs | 0 | 0 |
+-------------------------+-----------+------------+
| Worktable1 | | |
| logical reads regular | 1 924 136 | 8 200 130 |
| apf | 0 | 0 |
| total | 1 924 136 | 8 200 130 |
| physical reads regular | 1 621 916 | 11 906 846 |
| apf | 0 | 0 |
| total | 1 621 916 | 11 906 846 |
| apf IOs | 0 | 0 |
+-------------------------+-----------+------------+
| TableName | | |
| logical reads regular | 5 651 318 | 13 921 342 |
| apf | 52 | 20 |
| total | 5 651 370 | 13 921 362 |
| physical reads regular | 38 207 | 345 156 |
| apf | 820 646 | 1 768 064 |
| total | 858 853 | 2 113 220 |
| apf IOs | 819 670 | 1 754 171 |
+-------------------------+-----------+------------+
| Total writes | 0 | 5 675 198 |
| Execution time | 4 339 | 18 678 |
| CPU time | 211 321 | 930 657 |
| Elapsed time | 316 396 | 2 719 108 |
+-------------------------+-----------+------------+
5M rows:
Total writes for this command: 0
Execution Time 0.
Adaptive Server cpu time: 0 ms. Adaptive Server elapsed time: 0 ms.
Parse and Compile Time 0.
Adaptive Server cpu time: 0 ms.
Parse and Compile Time 0.
Adaptive Server cpu time: 0 ms.
Table: #AggTableName scan count 0, logical reads: (regular=81114 apf=0 total=81114), physical reads: (regular=0 apf=0 total=0), apf IOs used=0
Table: Worktable1 scan count 1, logical reads: (regular=1924136 apf=0 total=1924136), physical reads: (regular=1621916 apf=0 total=1621916), apf IOs used=0
Table: TableName scan count 1, logical reads: (regular=5651318 apf=52 total=5651370), physical reads: (regular=38207 apf=820646 total=858853), apf IOs used=819670
Total writes for this command: 0
Execution Time 4339.
Adaptive Server cpu time: 211321 ms. Adaptive Server elapsed time: 316396 ms.
13M rows:
Total writes for this command: 0
Execution Time 0.
Adaptive Server cpu time: 0 ms. Adaptive Server elapsed time: 0 ms.
Parse and Compile Time 1.
Adaptive Server cpu time: 50 ms.
Parse and Compile Time 0.
Adaptive Server cpu time: 0 ms.
Table: #AggTableName scan count 0, logical reads: (regular=248961 apf=0 total=248961), physical reads: (regular=2 apf=0 total=2), apf IOs used=0
Table: Worktable1 scan count 1, logical reads: (regular=8200130 apf=0 total=8200130), physical reads: (regular=11906846 apf=0 total=11906846), apf IOs used=0
Table: TableName scan count 1, logical reads: (regular=13921342 apf=20 total=13921362), physical reads: (regular=345156 apf=1768064 total=2113220), apf IOs used=1754171
Total writes for this command: 5675198
Execution Time 18678.
Adaptive Server cpu time: 930657 ms. Adaptive Server elapsed time: 2719108 ms.