January 2002
Intermediate to advanced
640 pages
16h 39m
English
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Explain
Shows the statement execution plan for a supplied query.
Synopsis
EXPLAIN [ VERBOSE ] queryParameters
VERBOSEThe optional verbose output keyword, which results in extra information being returned about a query plan.
queryThe query you intend to have explained.
Results
NOTICE: QUERY PLAN: planThe notice which will be followed by an explicit query plan sent from the backend.
EXPLAINThe message returned below the query plan, signifying that execution of the command is complete.
Description
Use the EXPLAIN command to view the execution plan for a query,
generated by PostgreSQL’s planner component. The planner component is the
part of PostgreSQL that attempts to determine the most efficient manner in which to execute a
SQL query. The execution plan details how tables referenced within your query will be scanned
by the database server. Depending on the circumstances, tables might be scanned sequentially,
or through the use of an index. The plan will list output for each table involved in the
execution plan.
The EXPLAIN command is useful for determining the relative
cost of query execution plans. This cost is measured literally in disk
page fetches. The more pages needed, the longer it takes a query to run.
PostgreSQL does not attempt to equate this number of fetches into a meaningful unit of time, as this will vary widely from machine to machine based on the hardware requirements and load of the operating system. The cost of a query execution plan is therefore only meaningful to the relative ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.