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doc/FAQ
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Frequently Asked Questions (FAQ) for PostgreSQL Frequently Asked Questions (FAQ) for PostgreSQL
Last updated: Thu Feb 14 12:14:47 EST 2002 Last updated: Mon Feb 18 00:08:54 EST 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) 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 As an example, consider a file of 100,000 lines with an integer and
text description on each line. Suppose the text string avergages text description on each line. Suppose the text string avergages
twenty characters in length. The flat file would be 2.8 MB. The size twenty bytes in length. The flat file would be 2.8 MB. The size of the
of the PostgreSQL database file containing this data can be estimated PostgreSQL database file containing this data can be estimated as 6.4
as 6.6 MB: MB:
36 bytes: each row header (approximate) 36 bytes: each row header (approximate)
26 bytes: two int fields @ 4 bytes each 24 bytes: one int field and one text filed
+ 4 bytes: pointer on page to tuple + 4 bytes: pointer on page to tuple
---------------------------------------- ----------------------------------------
66 bytes per row 64 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so: The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page 8192 bytes per page
------------------- = 124 rows per database page (rounded down) ------------------- = 128 rows per database page (rounded down)
66 bytes per row 64 bytes per row
100000 data rows 100000 data rows
-------------------- = 807 database pages (rounded up) -------------------- = 782 database pages (rounded up)
124 rows per page 128 rows per page
807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB) 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
Indexes do not require as much overhead, but do contain the data that Indexes do not require as much overhead, but do contain the data that
is being indexed, so they can be large also. is being indexed, so they can be large also.

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doc/src/FAQ/FAQ.html
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alink="#0000ff"> alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> <H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
<P>Last updated: Thu Feb 14 12:14:47 EST 2002</P> <P>Last updated: Mon Feb 18 00:08:54 EST 2002</P>
<P>Current maintainer: Bruce Momjian (<A href= <P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> "mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
@ -855,27 +855,27 @@
<P>As an example, consider a file of 100,000 lines with an integer <P>As an example, consider a file of 100,000 lines with an integer
and text description on each line. Suppose the text string avergages and text description on each line. Suppose the text string avergages
twenty characters in length. The flat file would be 2.8 MB. The size twenty bytes in length. The flat file would be 2.8 MB. The size
of the PostgreSQL database file containing this data can be of the PostgreSQL database file containing this data can be
estimated as 6.6 MB:</P> estimated as 6.4 MB:</P>
<PRE> <PRE>
36 bytes: each row header (approximate) 36 bytes: each row header (approximate)
26 bytes: two int fields @ 4 bytes each 24 bytes: one int field and one text filed
+ 4 bytes: pointer on page to tuple + 4 bytes: pointer on page to tuple
---------------------------------------- ----------------------------------------
66 bytes per row 64 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so: The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page 8192 bytes per page
------------------- = 124 rows per database page (rounded down) ------------------- = 128 rows per database page (rounded down)
66 bytes per row 64 bytes per row
100000 data rows 100000 data rows
-------------------- = 807 database pages (rounded up) -------------------- = 782 database pages (rounded up)
124 rows per page 128 rows per page
807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB) 782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
</PRE> </PRE>
<P>Indexes do not require as much overhead, but do contain the data <P>Indexes do not require as much overhead, but do contain the data