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>Chapter 57. Writing A Custom Scan Provider</H1
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>57.1. <A
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>Creating Custom Scan Paths</A
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>57.2. <A
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>Creating Custom Scan Plans</A
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>Executing Custom Scans</A
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>  <SPAN
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>PostgreSQL</SPAN
> supports a set of experimental facilities which
  are intended to allow extension modules to add new scan types to the system.
  Unlike a <A
HREF="fdwhandler.html"
>foreign data wrapper</A
>, which is only
  responsible for knowing how to scan its own foreign tables, a custom scan
  provider can provide an alternative method of scanning any relation in the
  system.  Typically, the motivation for writing a custom scan provider will
  be to allow the use of some optimization not supported by the core
  system, such as caching or some form of hardware acceleration.  This chapter
  outlines how to write a new custom scan provider.
 </P
><P
>  Implementing a new type of custom scan is a three-step process.  First,
  during planning, it is necessary to generate access paths representing a
  scan using the proposed strategy.  Second, if one of those access paths
  is selected by the planner as the optimal strategy for scanning a
  particular relation, the access path must be converted to a plan.
  Finally, it must be possible to execute the plan and generate the same
  results that would have been generated for any other access path targeting
  the same relation.
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