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Commitbc6fb54

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Update FAQ for new 24-byte header, down from 28.
1 parent1ad2f04 commitbc6fb54

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-18
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‎doc/FAQ‎

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated:Tue Feb6 22:16:17 EST 2007
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Last updated:Thu Feb8 22:43:13 EST 2007
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Current maintainer: Bruce Momjian (bruce@momjian.us)
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As an example, consider a file of 100,000 lines with an integer and
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text description on each line. Suppose the text string avergages
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twenty bytes in length. The flat file would be 2.8 MB. The size of the
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PostgreSQL database file containing this data can be estimated as 5.6
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PostgreSQL database file containing this data can be estimated as 5.2
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MB:
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28 bytes: each row header (approximate)
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24 bytes: each row header (approximate)
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24 bytes: one int field and one text field
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+ 4 bytes: pointer on page to tuple
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----------------------------------------
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56 bytes per row
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52 bytes per row
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The data page size in PostgreSQL is 8192 bytes (8 KB), so:
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8192 bytes per page
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------------------- =146 rows per database page (rounded down)
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56 bytes per row
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------------------- =158 rows per database page (rounded down)
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52 bytes per row
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100000 data rows
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-------------------- =685 database pages (rounded up)
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146 rows per page
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-------------------- =633 database pages (rounded up)
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158 rows per page
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685 database pages * 8192 bytes per page = 5,611,520 bytes (5.6 MB)
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633 database pages * 8192 bytes per page = 5,185,536 bytes (5.2 MB)
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Indexes do not require as much overhead, but do contain the data that
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is being indexed, so they can be large also.

‎doc/src/FAQ/FAQ.html‎

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alink="#0000ff">
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
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<P>Last updated:Tue Feb6 22:16:17 EST 2007</P>
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<P>Last updated:Thu Feb8 22:43:13 EST 2007</P>
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<P>Current maintainer: Bruce Momjian (<Ahref=
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"mailto:bruce@momjian.us">bruce@momjian.us</A>)
@@ -720,25 +720,25 @@ <H3 id="item4.5">4.5) How much database disk space is required
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and text description on each line. Suppose the text string
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avergages twenty bytes in length. The flat file would be 2.8 MB.
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The size of the PostgreSQL database file containing this data can
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be estimated as 5.6 MB:</P>
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be estimated as 5.2 MB:</P>
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<PRE>
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28 bytes: each row header (approximate)
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24 bytes: each row header (approximate)
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24 bytes: one int field and one text field
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+ 4 bytes: pointer on page to tuple
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----------------------------------------
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56 bytes per row
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52 bytes per row
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The data page size in PostgreSQL is 8192 bytes (8 KB), so:
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8192 bytes per page
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------------------- =146 rows per database page (rounded down)
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56 bytes per row
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------------------- =158 rows per database page (rounded down)
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52 bytes per row
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100000 data rows
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-------------------- =685 database pages (rounded up)
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146 rows per page
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-------------------- =633 database pages (rounded up)
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158 rows per page
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685 database pages * 8192 bytes per page = 5,611,520 bytes (5.6 MB)
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633 database pages * 8192 bytes per page = 5,185,536 bytes (5.2 MB)
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</PRE>
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<P>Indexes do not require as much overhead, but do contain the data

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