Postgres 8.4 性能提升

• Improve optimizer statistics calculations (Jan Urbanski, Tom) 优化统计计算。

  In particular, estimates for full-text-search operators are greatly improved. 特

  别是，全文搜索操作有极大的提升。

• Allow SELECT DISTINCT and UNION/INTERSECT/EXCEPT to use hashing (Tom) 允许 SELECT DISTINCT 和 UNION/INTERSECT/EXCEPT 使用 hash。

  This means that these types of queries no longer automatically produce sorted output.

  这意味着以上类型的查询不再进行排序（按我的理解，这使得查询操作并行化，特别对于分布到多个存储器上的数据库，性能会有提升明显——译者）。

• Create explicit concepts of semi-joins and anti-joins (Tom) 构造明确的半连接和反连接模式。

  This work formalizes our previous ad-hoc treatment of IN (SELECT ...) clauses, and extends it to EXISTS and NOT EXISTS clauses. It should result in significantly better planning of EXISTS and NOT EXISTS queries. In general, logically equivalent IN and EXISTS clauses should now have similar performance, whereas previously IN often won.

  这 使得 IN(SELECT ...) 子句，以及由此扩展的 EXISTS 和 NOT EXISTS 子句的分析正规化。更有利于规划 EXITS 和 NOT EXISTS 查询。通常来说， IN 和 EXISTS 子句在逻辑上等价。现在它们具有了近似的性能。而过去通常 IN 胜出。

• Improve optimization of sub-selects beneath outer joins (Tom) 优化了外连接之下的子查询性能。

  Formerly, a sub-select or view could not be optimized very well if it appeared within the nullable side of an outer join and contained non-strict expressions (for instance, constants) in its result list.

  以前，如果外连接可接受空值的那一边是子查询或者视图，并且返回列表里包含不严格的表达式（例如，常量），总是优化的不太好。

• Improve the performance of text_position() and related functions by using Boyer-Moore-Horspool searching (David Rowley) 提升了 text_position() 和使用 Boyer-Moore-Horspool 搜索的相关函数的性能。

  This is particularly helpful for long search patterns.

  这对于长搜索模式特别有用。

• Reduce I/O load of writing the statistics collection file by writing the file only when requested (Martin Pihlak) 减少了请求时写入文件造成的统计文件写入I/O。
• Improve performance for bulk inserts (Robert Haas, Simon) 优化了批量写入的性能。
• Increase the default value of default_statistics_target from 10 to 100 (Greg Sabino Mullane, Tom) 将 default_statistics_target 的默认值从10提升到一百。

  The maximum value was also increased from 1000 to 10000. 最大值也从 1000 提升到 10000。

• Perform constraint_exclusion checking by default in queries involving inheritance or UNION ALL (Tom) 默认的查询继承调用或者 UNION ALL 中的 constraint_exclusion 检查得到了优化。

  A new constraint_exclusion setting, partition, was added to specify this behavior.

  新的 constraint_exclusion 设置、分化加入到其特定的行为中。

• Allow I/O read-ahead for bitmap index scans (Greg Stark) 允许位图扫描 I/O 预读。

  The amount of read-ahead is controlled by effective_io_concurrency. This feature is available only if the kernel has posix_fadvise() support.

  预读计数由 effective_io_concurrency 控。这个功能只有内核支持 posix_fadvise() 才能使用。

• Inline simple set-returning SQL functions in FROM clauses (Richard Rowell) 内联 FORM 子句中的简单返回集合的 SQL 函数。
• Improve performance of multi-batch hash joins by providing a special case for join key values that are especially common in the outer relation (Bryce Cutt, Ramon Lawrence) 通过给定常见于外连接的，特定的连接关键值， 多路 hash 连接的性能得到优化。
• Reduce volume of temporary data in multi-batch hash joins by suppressing "physical tlist" optimization (Michael Henderson, Ramon Lawrence) 多路 hash 连接的临时数据卷通过“物理链表”得以缩减。
• Avoid waiting for idle-in-transaction sessions during CREATE INDEX CONCURRENTLY (Simon) 通过 CREATE INDEX CONCURRENTLY 避免了 idle-in-transaction 会话等待时间
• Improve performance of shared cache invalidation (Tom) 提升了共享缓存失效的性能。