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postgresql9.6-docs-9.6.22-1.mga7.noarch.rpm

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>Chapter 64. BRIN Indexes</TD
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>64.1. Introduction</A
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><P
>  <ACRONYM
CLASS="ACRONYM"
>BRIN</ACRONYM
> stands for Block Range Index.
  <ACRONYM
CLASS="ACRONYM"
>BRIN</ACRONYM
> is designed for handling very large tables
  in which certain columns have some natural correlation with their
  physical location within the table.
  A <I
CLASS="FIRSTTERM"
>block range</I
> is a group of pages that are physically
  adjacent in the table; for each block range, some summary info is stored
  by the index.
  For example, a table storing a store's sale orders might have
  a date column on which each order was placed, and most of the time
  the entries for earlier orders will appear earlier in the table as well;
  a table storing a ZIP code column might have all codes for a city
  grouped together naturally.
 </P
><P
>  <ACRONYM
CLASS="ACRONYM"
>BRIN</ACRONYM
> indexes can satisfy queries via regular bitmap
  index scans, and will return all tuples in all pages within each range if
  the summary info stored by the index is <I
CLASS="FIRSTTERM"
>consistent</I
> with the
  query conditions.
  The query executor is in charge of rechecking these tuples and discarding
  those that do not match the query conditions &mdash; in other words, these
  indexes are lossy.
  Because a <ACRONYM
CLASS="ACRONYM"
>BRIN</ACRONYM
> index is very small, scanning the index
  adds little overhead compared to a sequential scan, but may avoid scanning
  large parts of the table that are known not to contain matching tuples.
 </P
><P
>  The specific data that a <ACRONYM
CLASS="ACRONYM"
>BRIN</ACRONYM
> index will store,
  as well as the specific queries that the index will be able to satisfy,
  depend on the operator class selected for each column of the index.
  Data types having a linear sort order can have operator classes that
  store the minimum and maximum value within each block range, for instance;
  geometrical types might store the bounding box for all the objects
  in the block range.
 </P
><P
>  The size of the block range is determined at index creation time by
  the <TT
CLASS="LITERAL"
>pages_per_range</TT
> storage parameter.  The number of index
  entries will be equal to the size of the relation in pages divided by
  the selected value for <TT
CLASS="LITERAL"
>pages_per_range</TT
>.  Therefore, the smaller
  the number, the larger the index becomes (because of the need to
  store more index entries), but at the same time the summary data stored can
  be more precise and more data blocks can be skipped during an index scan.
 </P
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><A
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>64.1.1. Index Maintenance</A
></H2
><P
>   At the time of creation, all existing index pages are scanned and a
   summary index tuple is created for each range, including the
   possibly-incomplete range at the end.
   As new pages are filled with data, page ranges that are already
   summarized will cause the summary information to be updated with data
   from the new tuples.
   When a new page is created that does not fall within the last
   summarized range, that range does not automatically acquire a summary
   tuple; those tuples remain unsummarized until a summarization run is
   invoked later, creating initial summaries.
   This process can be invoked manually using the
   <CODE
CLASS="FUNCTION"
>brin_summarize_new_values(regclass)</CODE
> function,
   or automatically when <TT
CLASS="COMMAND"
>VACUUM</TT
> processes the table.
  </P
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