Wednesday, March 20, 2013

Parallelism in SAP Sybase ASE

Here're some random notes on the parallelism of SAP Sybase ASE.

The Parallel execution in ASE is implemented by the EXCHANGE operator. It marks the boundary between producer and consumer. Consumer process reads data from a pipe.

Parallelism implementations:

Table Scan:
For an unpartitioned table, hash-based table scan is used. Each work process examines 1/k rows of the page.
For partitioned-table, each partition is process by a work process

Scalar aggregation:
The aggregation operation can be performed in two phases or serial:
- Two-phase: using two scalar aggregate operators. The lower scalar aggregation operator performs aggregation on the data stream in parallel. The result of scalar aggregation is merged and aggregated a second time.
- Serial:  the result of the parallel scan is merged and aggregated.

Union all:

- parallel: each of its operands must be of the same degree, then union all is done in parallel, and result is merged.
- serial: when restricted by selective predicates, the amount of data being sent through the union all operator is small enough, and only scan is done in parallel. The merged scan result is send to union all.

Join:
- two tables with same useful partitioning
This is called an equipartitioned join. Joins are done in each partition in parallel, then merged. (nested loop join is often used)

- One of the tables(tb2) with useful partitioning
The query optimizer dynamically repartitions the table (tb1) to match the partitioning of tb2, by using hash partitioning on the join column. So the 1st exchange operator does the parallel repartition of tb1. The 2nd exchange operator does the join. (merge join is often used)

- Both tables with useless partitioning
Both tables will be repartitioned. The repartitioned operands of the join are equipartitioned with respect to the join predicate. (hash join is often used)

- Replicated join:
When a large table has a useful index on the joining column, but useless partitioning, and joins to a small table. The small table can be replicated N ways to that of the inner table, where N is the number of partitions of the large table. Each partition of the large table is joined with the small table and, because no exchange operator is needed on the inner side of the join, an index nested-loop join is allowed.

- Parallel reformatting
When there is no useful index on the joining column or nested-loop join is the only viable option.
The outer side may have useful partitioning or we can repartitioned to create that. But for the inner side of a nested-loop join, any repartitioning means that the table must be reformatted into a worktable that uses the new partitioning strategy, then then creating an index on the joining predicate. The inner scan of a nested-loop join then access the worktable.

Vector aggregation (group-by):
- In-partitioned vector aggregation
If grouping is partitioned on a subset, or on the same columns as that of the columns in the group by clause, the grouping operation can be done in parallel on each of the partitions.

- Repartitioned vector aggregation
If the the partitioning is not useful, repartitioning the source data to match the grouping columns, then applying the parallel vector aggregation.

Distinct
same as group by.

Queries with an in list
in (...) list is transformed into an or list. The values in the in list are put into a special in-memory table and sorted for removal of duplicates.The table is then joined back with the base table using an index nested-loop join.

Queries with or clauses
The set of conjunctive predicates on each side of the disjunction must be indexable. We apply each side of the disjunction separately to qualify a set of row IDs (RIDs). We can use different indexes here. The predicates may qualify an overlapping set of data rows. These row IDs are then merged to duplicate elimination. Finally, RIDs are joined back to the base table to get the results.

Queries with an order by clause
If no inherent ordering is available, it needs to repartition an existing data stream or it may use the existing partitioning scheme, then apply the sort to each of the constituent streams. Result is merged.

Select into clauses
- Creates the new table using the columns specified in the select into statement.
- Creates N partitions in the new table, where N is the degree of parallelism that the optimizer chooses for the insert operation in the query.
- Populates the new table with query results, using N worker processes.
- Unpartitions the new table, if no specific destination partitioning is required.
If destination partitioning is specified:
- If the destination table has the same partition as the source data, and there is enough data to insert, the insert operator executes in parallel.
- If the source partitioning does not match that of the destination table, the source data must be repartitioned. Insert is done in parallel.

insert/delete/update
insert, delete, and update operations are done in serial.
However, tables other than the destination table can be accessed in parallel.

Partition elimination
Query processor is able to disqualify range, hash, and list partitions at compile time. With hash partitions, only equality predicates can be used, whereas for range and list partitions, equality and in-equality predicates can be used to eliminate partitions.

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