Common Causes of Performance Issues
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Common Causes of Performance Issues
This section explains the troubleshooting processes for common performance issues and potential solutions to these issues.
Identifying Hardware and Segment Failures
The performance of Greenplum Database depends on the hardware and IT infrastructure on which it runs. Greenplum Database is comprised of several servers (hosts) acting together as one cohesive system (array); as a first step in diagnosing performance problems, ensure that all Greenplum Database segments are online. Greenplum Database's performance will be as fast as the slowest host in the array. Problems with CPU utilization, memory management, I/O processing, or network load affect performance. Common hardware-related issues are:
- Disk Failure – Although a single disk failure should not dramatically affect database performance if you are using RAID, disk resynchronization does consume resources on the host with failed disks. The gpcheckperf utility can help identify segment hosts that have disk I/O issues.
- Host Failure – When a host is offline, the segments on that host are nonoperational. This means other hosts in the array must perform twice their usual workload because they are running the primary segments and multiple mirrors. If mirrors are not enabled, service is interrupted. Service is temporarily interrupted to recover failed segments. The gpstate utility helps identify failed segments.
- Network Failure – Failure of a network interface card, a switch, or DNS server can bring down segments. If host names or IP addresses cannot be resolved within your Greenplum array, these manifest themselves as interconnect errors in Greenplum Database. The gpcheckperf utility helps identify segment hosts that have network issues.
Disk Capacity – Disk capacity on your segment hosts should never
exceed 70 percent full. Greenplum Database needs some free space for runtime
processing. To reclaim disk space that deleted rows occupy, run
VACUUM after loads or updates. The gp_toolkit
administrative schema has many views for checking the size of distributed database
See the Greenplum Database Reference Guide for information about checking database object sizes and disk space.
A database system has a limited CPU capacity, memory, and disk I/O resources. When multiple workloads compete for access to these resources, database performance suffers. Workload management maximizes system throughput while meeting varied business requirements. With role-based resource queues, Greenplum Database workload management limits active queries and conserves system resources.
A resource queue limits the size and/or total number of queries that users or roles can execute in the particular queue. By assigning database roles to the appropriate resource queues, administrators can control concurrent user queries and prevent system overload. See Workload Management with Resource Queues for more information about setting up resource queues.
Greenplum Database administrators should run maintenance workloads such as data loads and VACUUM ANALYZE operations after business hours. Do not compete with database users for system resources; perform administrative tasks at low-usage times.
Contention arises when multiple users or workloads try to use the system in a conflicting way; for example, contention occurs when two transactions try to update a table simultaneously. A transaction that seeks a table-level or row-level lock will wait indefinitely for conflicting locks to be released. Applications should not hold transactions open for long periods of time, for example, while waiting for user input.
Maintaining Database Statistics
Identifying Statistics Problems in Query Plans
Before you interpret a query plan for a query using EXPLAIN or EXPLAIN ANALYZE, familiarize yourself with the data to help identify possible statistics problems. Check the plan for the following indicators of inaccurate statistics:
- Are the optimizer's estimates close to reality? Run EXPLAIN ANALYZE and see if the number of rows the optimizer estimated is close to the number of rows the query operation returned .
- Are selective predicates applied early in the plan? The most selective filters should be applied early in the plan so fewer rows move up the plan tree.
- Is the optimizer choosing the best join order? When you have a query that joins multiple tables, make sure the optimizer chooses the most selective join order. Joins that eliminate the largest number of rows should be done earlier in the plan so fewer rows move up the plan tree.
See Query Profiling for more information about reading query plans.
Tuning Statistics Collection
The following configuration parameters control the amount of data sampled for statistics collection:
These parameters control statistics sampling at the system level. It is better to sample only increased statistics for columns used most frequently in query predicates. You can adjust statistics for a particular column using the command:
ALTER TABLE...SET STATISTICS
ALTER TABLE sales ALTER COLUMN region SET STATISTICS 50;
This is equivalent to increasing default_statistics_target for a particular column. Subsequent ANALYZE operations will then gather more statistics data for that column and produce better query plans as a result.
Optimizing Data Distribution
When you create a table in Greenplum Database, you must declare a distribution key that allows for even data distribution across all segments in the system. Because the segments work on a query in parallel, Greenplum Database will always be as fast as the slowest segment. If the data is unbalanced, the segments that have more data will return their results slower and therefore slow down the entire system.
Optimizing Your Database Design
Many performance issues can be improved by database design. Examine your database design and consider the following:
- Does the schema reflect the way the data is accessed?
- Can larger tables be broken down into partitions?
- Are you using the smallest data type possible to store column values?
- Are columns used to join tables of the same datatype?
- Are your indexes being used?
Greenplum Database Maximum Limits
To help optimize database design, review the maximum limits that Greenplum Database supports:
|Table Size||Unlimited, 128 TB per partition per segment|
|Row Size||1.6 TB (1600 columns * 1 GB)|
|Field Size||1 GB|
|Rows per Table||281474976710656 (2^48)|
|Columns per Table/View||1600|
|Indexes per Table||Unlimited|
|Columns per Index||32|
|Table-level Constraints per Table||Unlimited|
|Table Name Length||63 Bytes (Limited by name data type)|
Dimensions listed as unlimited are not intrinsically limited by Greenplum Database. However, they are limited in practice to available disk space and memory/swap space. Performance may suffer when these values are unusually large.
There is a maximum limit on the number of objects (tables, indexes, and views, but not rows) that may exist at one time. This limit is 4294967296 (2^32).