There are limitations in Greenplum Database when using the default GPORCA optimizer. GPORCA and the Postgres Planner currently coexist in Greenplum Database because GPORCA does not support all Greenplum Database features.
Unsupported SQL Query Features
Certain query features are not supported with the default GPORCA optimizer. When an unsupported query is executed, Greenplum logs this notice along with the query text:
Feature not supported by the Greenplum Query Optimizer: UTILITY command
These features are unsupported when GPORCA is enabled (the default):
- Prepared statements that have parameterized values.
- Indexed expressions (an index defined as expression based on one or more columns of the table)
- SP-GiST indexing method. GPORCA supports only B-tree, bitmap, GIN, and GiST indexes. GPORCA ignores indexes created with unsupported methods.
- External parameters
- These types of partitioned tables:
- Non-uniform partitioned tables.
- Partitioned tables that have been altered to use an external table as a leaf child partition.
- SortMergeJoin (SMJ).
- Ordered aggregations.
- These analytics extensions:
- Multiple grouping sets
- These scalar operators:
- Aggregate functions that take set operators as input arguments.
- Multiple Distinct Qualified Aggregates, such as SELECT count(DISTINCT a), sum(DISTINCT b) FROM foo, are not supported by default. They can be enabled with the optimizer_enable_multiple_distinct_aggs Configuration Parameter.
- percentile_* window functions (ordered-set aggregate functions).
- Inverse distribution functions.
- Queries that execute functions that are defined with the ON MASTER or ON ALL SEGMENTS attribute.
- Queries that contain UNICODE characters in metadata names, such as table names, and the characters are not compatible with the host system locale.
- SELECT, UPDATE, and DELETE commands where a table name is qualified by the ONLY keyword.
- Per-column collation. GPORCA supports collation only when all columns in the query use the same collation. If columns in the query use different collations, then Greenplum uses the Postgres Planner.
The following features are known performance regressions that occur with GPORCA enabled:
- Short running queries - For GPORCA, short running queries might encounter additional overhead due to GPORCA enhancements for determining an optimal query execution plan.
- ANALYZE - For GPORCA, the ANALYZE command generates root partition statistics for partitioned tables. For the Postgres Planner, these statistics are not generated.
- DML operations - For GPORCA, DML enhancements including the support of updates on partition and distribution keys might require additional overhead.
Also, enhanced functionality of the features from previous versions could result in additional time required when GPORCA executes SQL statements with the features.