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体验新版 GitCode,发现更多精彩内容 >>
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fb814708
编写于
5月 08, 2019
作者:
D
David Yozie
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add notice about what cost means in explain output (#7631)
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gpdb-doc/dita/admin_guide/query/topics/query-profiling.xml
gpdb-doc/dita/admin_guide/query/topics/query-profiling.xml
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gpdb-doc/dita/admin_guide/query/topics/query-profiling.xml
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@@ -59,7 +59,16 @@
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@@ -59,7 +59,16 @@
sequential disk page read. The first estimate is the start-up cost of getting the first
sequential disk page read. The first estimate is the start-up cost of getting the first
row and the second is the total cost of cost of getting all rows. The total cost assumes
row and the second is the total cost of cost of getting all rows. The total cost assumes
all rows will be retrieved, which is not always true; for example, if the query uses
all rows will be retrieved, which is not always true; for example, if the query uses
<codeph>
LIMIT
</codeph>
, not all rows are retrieved.
</li>
<codeph>
LIMIT
</codeph>
, not all rows are retrieved.
<note>
The cost values generated by
the Pivotal Query Optimizer and the Postgres Planner are not directly comparable. The
two optimizers use different cost models, as well as different algorithms, to determine
the cost of an execution plan. Nothing can or should be inferred by comparing cost
values between the two optimizers.
<p>
In addition, the cost generated for any given
optimizer is valid only for comparing plan alternatives for a given single query and
set of statistics. Different queries can generate plans with different costs, even
when keeping the optimizer a constant.
</p><p>
To summarize, the cost is essentially an
internal number used by a given optimizer, and nothing should be inferred by examining
only the cost value displayed in the
<codeph>
EXPLAIN
</codeph>
plans.
</p></note></li>
<li
id=
"in182483"
><b>
rows
</b>
—The total number of rows output by this plan node. This
<li
id=
"in182483"
><b>
rows
</b>
—The total number of rows output by this plan node. This
number is usually less than the number of rows processed or scanned by the plan node,
number is usually less than the number of rows processed or scanned by the plan node,
reflecting the estimated selectivity of any
<codeph>
WHERE
</codeph>
clause conditions.
reflecting the estimated selectivity of any
<codeph>
WHERE
</codeph>
clause conditions.
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