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我是
postgresql的新手,我试着理解
explain(缓冲区,分析)指令.我有一个查询,我使用explain(buffers,analyze)执行它.
第一次执行它时性能比第二次差.此外,第一次在’hit’旁边获得’read’参数,而第二次’read’不存在.
有人可以帮我理解吗？
解决方法
第一次选择,页面变热 – 它们被加载到缓存,一旦它们在RAM中 – 所有下一个选择将更快(RAM速度更高).
因此缓冲区显示读取,当页面不在缓存中时,cos postgres读取它们,并且当它们变热时不读取,因此缓存被命中…
使用docs更新：
BUFFERS
Include information on buffer usage. Specifically,include the
number of shared blocks hit,read,dirtied,and written,the number of
local blocks hit,and the number of temp
blocks read and written. A hit means that a read was avoided because
the block was found already in cache when needed. Shared blocks
contain data from regular tables and indexes; local blocks contain
data from temporary tables and indexes; while temp blocks contain
short-term working data used in sorts,hashes,Materialize plan nodes,
and similar cases. The number of blocks dirtied indicates the number
of previously unmodified blocks that were changed by this query; while
the number of blocks written indicates the number of
previously-dirtied blocks evicted from cache by this backend during
query processing. The number of blocks shown for an upper-level node
includes those used by all its child nodes. In text format,only
non-zero values are printed. This parameter may only be used when
ANALYZE is also enabled. It defaults to FALSE.
令人惊讶的是缓冲区here并不多.
（编辑：李大同）
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