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Commit865d26f

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Update FAQ.
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‎doc/FAQ

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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated:Thu Feb14 12:14:47 EST 2002
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Last updated:Mon Feb18 00:08:54 EST 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.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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twentycharacters in length. The flat file would be 2.8 MB. The size
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of thePostgreSQL database file containing this data can be estimated
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as 6.6MB:
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twentybytes 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 6.4
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MB:
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36 bytes: each row header (approximate)
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26 bytes:two intfields @ 4 bytes each
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24 bytes:one intfield and one text filed
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+ 4 bytes: pointer on page to tuple
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----------------------------------------
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66 bytes per row
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64 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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------------------- =124 rows per database page (rounded down)
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66 bytes per row
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------------------- =128 rows per database page (rounded down)
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64 bytes per row
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100000 data rows
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-------------------- =807 database pages (rounded up)
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124 rows per page
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-------------------- =782 database pages (rounded up)
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128 rows per page
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807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
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782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 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:Thu Feb14 12:14:47 EST 2002</P>
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<P>Last updated:Mon Feb18 00:08:54 EST 2002</P>
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<P>Current maintainer: Bruce Momjian (<Ahref=
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@@ -855,27 +855,27 @@ <H4><A name="4.6">4.6</A>) How much database disk space is required
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<P>As an example, consider a file of 100,000 lines with an integer
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and text description on each line. Suppose the text string avergages
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twentycharacters in length. The flat file would be 2.8 MB. The size
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twentybytes in length. The flat file would be 2.8 MB. The size
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of the PostgreSQL database file containing this data can be
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estimated as 6.6 MB:</P>
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estimated as 6.4 MB:</P>
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<PRE>
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36 bytes: each row header (approximate)
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26 bytes:two intfields @ 4 bytes each
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24 bytes:one intfield and one text filed
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+ 4 bytes: pointer on page to tuple
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----------------------------------------
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66 bytes per row
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64 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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------------------- =124 rows per database page (rounded down)
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66 bytes per row
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------------------- =128 rows per database page (rounded down)
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64 bytes per row
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100000 data rows
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-------------------- =807 database pages (rounded up)
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124 rows per page
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-------------------- =782 database pages (rounded up)
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128 rows per page
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807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
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782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 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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