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@ -10,7 +10,7 @@ |
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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: Sat Apr 23 14:59:01 EDT 2005</P> |
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<P>Last updated: Sat Apr 23 16:49:43 EDT 2005</P> |
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<P>Current maintainer: Bruce Momjian (<A href= |
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>) |
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@ -99,36 +99,24 @@ |
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reused on transaction abort? Why are there gaps in the numbering of |
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my sequence/SERIAL column?<BR> |
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<A href="#4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is a |
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<SMALL>TID</SMALL>?<BR> |
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<A href="#4.12">4.13</A>) Why do I get the error <I>"ERROR: Memory |
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<SMALL>CTID</SMALL>?<BR> |
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<A href="#4.13">4.13</A>) Why do I get the error <I>"ERROR: Memory |
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exhausted in AllocSetAlloc()"</I>?<BR> |
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<A href="#4.14">4.14</A>) How do I tell what PostgreSQL version I |
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am running?<BR> |
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<A href="#4.15">4.15</A>) Why does my large-object operations get |
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<I>"invalid large obj descriptor"</I>?<BR> |
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<A href="#4.16">4.16</A>) How do I create a column that will |
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<A href="#4.15">4.15</A>) How do I create a column that will |
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default to the current time?<BR> |
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<A href="#4.17">4.17</A>) How do I perform an outer join?<BR> |
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<A href="#4.18">4.18</A>) How do I perform queries using multiple |
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<A href="#4.16">4.16</A>) How do I perform an outer join?<BR> |
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<A href="#4.17">4.17</A>) How do I perform queries using multiple |
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databases?<BR> |
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<A href="#4.19">4.19</A>) How do I return multiple rows or columns |
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<A href="#4.18">4.18</A>) How do I return multiple rows or columns |
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from a function?<BR> |
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<A href="#4.20">4.20</A>) Why do I get "relation with OID ##### |
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<A href="#4.19">4.19</A>) Why do I get "relation with OID ##### |
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does not exist" errors when accessing temporary tables in PL/PgSQL |
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functions?<BR> |
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<A href="#4.21">4.21</A>) What encryption options are available?<BR> |
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<A href="#4.20">4.20</A>) What encryption options are available?<BR> |
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<H2 align="center">Extending PostgreSQL</H2> |
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<A href="#5.1">5.1</A>) I wrote a user-defined function. When I run |
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it in <I>psql</I>, why does it dump core?<BR> |
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<A href="#5.2">5.2</A>) How can I contribute some nifty new types |
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and functions to PostgreSQL?<BR> |
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<A href="#5.3">5.3</A>) How do I write a C function to return a |
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tuple?<BR> |
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<A href="#5.4">5.4</A>) I have changed a source file. Why does the |
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recompile not see the change?<BR> |
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<HR> |
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<H2 align="center">General Questions</H2> |
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@ -683,7 +671,7 @@ table?</TD><TD>unlimited</TD></TR> |
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<P>If you believe the optimizer is incorrect in choosing a |
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sequential scan, use <CODE>SET enable_seqscan TO 'off'</CODE> and |
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run tests to see if an index scan is indeed faster.</P> |
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run query again to see if an index scan is indeed faster.</P> |
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<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or |
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<I>~</I>, indexes can only be used in certain circumstances:</P> |
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@ -733,7 +721,6 @@ table?</TD><TD>unlimited</TD></TR> |
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FROM tab |
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WHERE lower(col) = 'abc'; |
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</PRE> |
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This will not use an standard index. However, if you create a |
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expresssion index, it will be used: |
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<PRE> |
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@ -745,7 +732,7 @@ table?</TD><TD>unlimited</TD></TR> |
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NULL</SMALL> or not?</H3> |
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<P>You test the column with <SMALL>IS NULL</SMALL> and <SMALL>IS |
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NOT NULL</SMALL>.</P> |
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NOT NULL</SMALL>, like this:</P> |
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<PRE> |
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SELECT * |
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@ -754,7 +741,7 @@ table?</TD><TD>unlimited</TD></TR> |
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</PRE> |
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<P>To sort by the <SMALL>NULL</SMALL> status, use the <SMALL>IS NULL</SMALL> |
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and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>WHERE</SMALL> clause. |
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and <SMALL>IS NOT NULL</SMALL> modifiers in your <SMALL>ORDER BY</SMALL> clause. |
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Things that are <I>true</I> will sort higher than things that are <I>false</I>, |
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so the following will put NULL entries at the top of the resulting list:</P> |
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@ -787,9 +774,8 @@ length</TD></TR> |
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<P>The first four types above are "varlena" types (i.e., the first |
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four bytes on disk are the length, followed by the data). Thus the |
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actual space used is slightly greater than the declared size. |
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However, these data types are also subject to compression or being |
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stored out-of-line by <SMALL>TOAST</SMALL>, so the space on disk |
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might also be less than expected.</P> |
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However, long values are also subject to compression, so the space |
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on disk might also be less than expected.</P> |
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<SMALL>VARCHAR(n)</SMALL> is best when storing variable-length |
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strings and it limits how long a string can be. <SMALL>TEXT</SMALL> |
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@ -805,8 +791,7 @@ length</TD></TR> |
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serial/auto-incrementing field?</H3> |
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<P>PostgreSQL supports a <SMALL>SERIAL</SMALL> data type. It |
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auto-creates a sequence. For example, |
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this:</P> |
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auto-creates a sequence. For example, this:</P> |
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<PRE> |
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CREATE TABLE person ( |
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id SERIAL, |
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@ -815,11 +800,12 @@ length</TD></TR> |
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</PRE> |
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is automatically translated into this: |
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<PRE> |
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CREATE SEQUENCE person_id_seq; |
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CREATE TABLE person ( |
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id INT4 NOT NULL DEFAULT nextval('person_id_seq'), |
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name TEXT |
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name TEXT |
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); |
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</PRE> |
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@ -839,17 +825,17 @@ length</TD></TR> |
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execute("INSERT INTO person (id, name) VALUES (new_id, 'Blaise Pascal')"); |
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</PRE> |
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You would then also have the new value stored in |
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<CODE>new_id</CODE> for use in other queries (e.g., as a foreign |
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key to the <CODE>person</CODE> table). Note that the name of the |
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automatically created <SMALL>SEQUENCE</SMALL> object will be named |
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<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where |
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<I>table</I> and <I>serialcolumn</I> are the names of your table |
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and your <SMALL>SERIAL</SMALL> column, respectively. |
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<P>Alternatively, you could retrieve the assigned |
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<SMALL>SERIAL</SMALL> value with the <I>currval()</I> function |
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<I>after</I> it was inserted by default, e.g.,</P> |
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You would then also have the new value stored in <CODE>new_id</CODE> |
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for use in other queries (e.g., as a foreign key to the <CODE>person |
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</CODE> table). Note that the name of the automatically created |
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<SMALL>SEQUENCE</SMALL> object will be named <<I>table</I>>_<<I> |
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serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I> |
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are the names of your table and your <SMALL>SERIAL</SMALL> column, |
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respectively. |
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<P>Alternatively, you could retrieve the assigned <SMALL>SERIAL</SMALL> |
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value with the <I>currval()</I> function <I>after</I> it was inserted by |
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default, e.g.,</P> |
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<PRE> |
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execute("INSERT INTO person (name) VALUES ('Blaise Pascal')"); |
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new_id = execute("SELECT currval('person_id_seq')"); |
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@ -871,7 +857,7 @@ length</TD></TR> |
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transactions.</P> |
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<H3><A name="4.12">4.12</A>) What is an <SMALL>OID</SMALL>? What is |
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a <SMALL>TID</SMALL>?</H3> |
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a <SMALL>CTID</SMALL>?</H3> |
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<P>Every row that is created in PostgreSQL gets a unique |
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<SMALL>OID</SMALL> unless created <SMALL>WITHOUT OIDS</SMALL>. |
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@ -888,8 +874,8 @@ length</TD></TR> |
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<SMALL>SERIAL8</SMALL> is available for storing eight-byte sequence |
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values.</P> |
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<P>T<SMALL>ID</SMALL>s are used to identify specific physical rows |
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with block and offset values. T<SMALL>ID</SMALL>s change after rows |
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<P>C<SMALL>TID</SMALL>s are used to identify specific physical rows |
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with block and offset values. C<SMALL>TID</SMALL>s change after rows |
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are modified or reloaded. They are used by index entries to point |
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to physical rows.</P> |
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@ -917,23 +903,7 @@ length</TD></TR> |
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<P>From <I>psql</I>, type <CODE>SELECT version();</CODE></P> |
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<H3><A name="4.15">4.15</A>) Why does my large-object operations |
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get <I>"invalid large obj descriptor"</I>?</H3> |
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<P>You need to put <CODE>BEGIN WORK</CODE> and <CODE>COMMIT</CODE> |
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around any use of a large object handle, that is, surrounding |
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<CODE>lo_open</CODE> ... <CODE>lo_close.</CODE></P> |
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<P>Currently PostgreSQL enforces the rule by closing large object |
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handles at transaction commit. So the first attempt to do anything |
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with the handle will draw <I>invalid large obj descriptor</I>. So |
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code that used to work (at least most of the time) will now |
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generate that error message if you fail to use a transaction.</P> |
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<P>If you are using a client interface like <SMALL>ODBC</SMALL> you |
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may need to set <CODE>auto-commit off.</CODE></P> |
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<H3><A name="4.16">4.16</A>) How do I create a column that will |
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<H3><A name="4.15">4.15</A>) How do I create a column that will |
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default to the current time?</H3> |
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<P>Use <I>CURRENT_TIMESTAMP</I>:</P> |
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@ -941,7 +911,7 @@ length</TD></TR> |
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CREATE TABLE test (x int, modtime TIMESTAMP DEFAULT CURRENT_TIMESTAMP ); |
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</PRE> |
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<H3><A name="4.17">4.17</A>) How do I perform an outer join?</H3> |
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<H3><A name="4.16">4.16</A>) How do I perform an outer join?</H3> |
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<P>PostgreSQL supports outer joins using the SQL standard syntax. |
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Here are two examples:</P> |
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@ -964,7 +934,7 @@ length</TD></TR> |
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<SMALL>RIGHT</SMALL>, and <SMALL>FULL</SMALL> joins. Ordinary joins |
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are called <SMALL>INNER</SMALL> joins.</P> |
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<H3><A name="4.18">4.18</A>) How do I perform queries using |
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<H3><A name="4.17">4.17</A>) How do I perform queries using |
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multiple databases?</H3> |
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<P>There is no way to query a database other than the current one. |
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@ -976,14 +946,14 @@ length</TD></TR> |
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connections to different databases and merge the results on the |
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client side.</P> |
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<H3><A name="4.19">4.19</A>) How do I return multiple rows or |
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<H3><A name="4.18">4.18</A>) How do I return multiple rows or |
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columns from a function?</H3> |
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<P>It is easy using set-returning functions, |
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<a href="http://techdocs.postgresql.org/guides/SetReturningFunctions"> |
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http://techdocs.postgresql.org/guides/SetReturningFunctions</a></P>. |
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<H3><A name="4.20">4.20</A>) Why do I get "relation with OID ##### |
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<H3><A name="4.19">4.19</A>) Why do I get "relation with OID ##### |
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does not exist" errors when accessing temporary tables in PL/PgSQL |
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functions?</H3> |
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@ -995,7 +965,7 @@ length</TD></TR> |
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<SMALL>EXECUTE</SMALL> for temporary table access in PL/PgSQL. This |
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will cause the query to be reparsed every time.</P> |
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<H3><A name="4.21">4.21</A>) What encryption options are available? |
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<H3><A name="4.20">4.20</A>) What encryption options are available? |
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</H3> |
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<UL> |
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<LI><I>contrib/pgcrypto</I> contains many encryption functions for |
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@ -1009,42 +979,8 @@ length</TD></TR> |
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native SSL connections.)</LI> |
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<LI>Database user passwords are automatically encrypted when stored in |
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the system tables.</LI> |
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<LI>The server can run using an encrypted file system.</LI> |
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<LI>The server can also run using an encrypted file system.</LI> |
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</UL> |
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<HR> |
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<H2 align="center">Extending PostgreSQL</H2> |
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<H3><A name="5.1">5.1</A>) I wrote a user-defined function. When I |
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run it in <I>psql</I>, why does it dump core?</H3> |
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<P>The problem could be a number of things. Try testing your |
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user-defined function in a stand-alone test program first.</P> |
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<H3><A name="5.2">5.2</A>) How can I contribute some nifty new |
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types and functions to PostgreSQL?</H3> |
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<P>Send your extensions to the <I>pgsql-hackers</I> mailing list, |
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and they will eventually end up in the <I>contrib/</I> |
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subdirectory.</P> |
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<H3><A name="5.3">5.3</A>) How do I write a C function to return a |
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tuple?</H3> |
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<P>In versions of PostgreSQL beginning with 7.3, table-returning |
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functions are fully supported in C, PL/PgSQL, and SQL. See the |
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Programmer's Guide for more information. An example of a |
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table-returning function defined in C can be found in |
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<I>contrib/tablefunc</I>.</P> |
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<H3><A name="5.4">5.4</A>) I have changed a source file. Why does |
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the recompile not see the change?</H3> |
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<P>The <I>Makefiles</I> do not have the proper dependencies for |
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include files. You have to do a <I>make clean</I> and then another |
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<I>make</I>. If you are using <SMALL>GCC</SMALL> you can use the |
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<I>--enable-depend</I> option of <I>configure</I> to have the |
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compiler compute the dependencies automatically.</P> |
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</BODY> |
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</HTML> |
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Bruce Momjian
21 years ago