Update FAQ.
This commit is contained in:
Bruce Momjian
parent
1ba857513d
commit
bc11dd61d3
42
doc/FAQ
42
doc/FAQ
|
|
@ -691,7 +691,7 @@ Maximum number of indexes on a table? unlimited
|
|||
4.7)How much database disk space is required to store data from a typical
|
||||
text file?
|
||||
|
||||
A PostgreSQL database may need six and a half times the disk space
|
||||
A PostgreSQL database may need six-and-a-half times the disk space
|
||||
required to store the data in a flat file.
|
||||
|
||||
Consider a file of 300,000 lines with two integers on each line. The
|
||||
|
|
@ -738,7 +738,7 @@ Maximum number of indexes on a table? unlimited
|
|||
faster.
|
||||
|
||||
For column-specific optimization statistics, use VACUUM ANALYZE.
|
||||
VACUUM ANALYZE is important for complex multi-join queries, so the
|
||||
VACUUM ANALYZE is important for complex multijoin queries, so the
|
||||
optimizer can estimate the number of rows returned from each table,
|
||||
and choose the proper join order. The backend does not keep track of
|
||||
column statistics on its own, so VACUUM ANALYZE must be run to collect
|
||||
|
|
@ -763,34 +763,34 @@ Maximum number of indexes on a table? unlimited
|
|||
handle range searches. A B-tree index only handles range searches in a
|
||||
single dimension. R-tree's can handle multi-dimensional data. For
|
||||
example, if an R-tree index can be built on an attribute of type
|
||||
point, the system can more efficient answer queries like select all
|
||||
points within a bounding rectangle.
|
||||
point, the system can more efficiently answer queries such as "select
|
||||
all points within a bounding rectangle."
|
||||
|
||||
The canonical paper that describes the original R-Tree design is:
|
||||
The canonical paper that describes the original R-tree design is:
|
||||
|
||||
Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial
|
||||
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial
|
||||
Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data,
|
||||
45-57.
|
||||
|
||||
You can also find this paper in Stonebraker's "Readings in Database
|
||||
Systems"
|
||||
|
||||
Builtin R-Trees can handle polygons and boxes. In theory, R-trees can
|
||||
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
|
||||
be extended to handle higher number of dimensions. In practice,
|
||||
extending R-trees require a bit of work and we don't currently have
|
||||
extending R-trees requires a bit of work and we don't currently have
|
||||
any documentation on how to do it.
|
||||
|
||||
4.12) What is Genetic Query Optimization?
|
||||
|
||||
The GEQO module speeds query optimization when joining many tables by
|
||||
means of a Genetic Algorithm (GA). It allows the handling of large
|
||||
join queries through non-exhaustive search.
|
||||
join queries through nonexhaustive search.
|
||||
|
||||
4.13) How do I do regular expression searches and case-insensitive regular
|
||||
expression searches?
|
||||
|
||||
The ~ operator does regular-expression matching, and ~* does
|
||||
case-insensitive regular-expression matching. There is no
|
||||
The ~ operator does regular expression matching, and ~* does
|
||||
case-insensitive regular expression matching. There is no
|
||||
case-insensitive variant of the LIKE operator, but you can get the
|
||||
effect of case-insensitive LIKE with this:
|
||||
WHERE lower(textfield) LIKE lower(pattern)
|
||||
|
|
@ -812,7 +812,7 @@ BYTEA bytea variable-length array of bytes
|
|||
You will see the internal name when examining system catalogs and in
|
||||
some error messages.
|
||||
|
||||
The last four types above are "varlena" types (i.e. the first four
|
||||
The last four types above are "varlena" types (i.e., the first four
|
||||
bytes are the length, followed by the data). char(#) allocates the
|
||||
maximum number of bytes no matter how much data is stored in the
|
||||
field. text, varchar(#), and bytea all have variable length on the
|
||||
|
|
@ -855,17 +855,17 @@ BYTEA bytea variable-length array of bytes
|
|||
|
||||
You would then also have the new value stored in $newSerialID for use
|
||||
in other queries (e.g., as a foreign key to the person table). Note
|
||||
that the name of the automatically-created SEQUENCE object will be
|
||||
that the name of the automatically created SEQUENCE object will be
|
||||
named <table>_<serialcolumn>_seq, where table and serialcolumn are the
|
||||
names of your table and your SERIAL column, respectively.
|
||||
|
||||
Alternatively, you could retrieve the just-assigned SERIAL value with
|
||||
the currval() function after it was inserted by default, e.g.,
|
||||
Alternatively, you could retrieve the assigned SERIAL value with the
|
||||
currval() function after it was inserted by default, e.g.,
|
||||
INSERT INTO person (name) VALUES ('Blaise Pascal');
|
||||
$newID = currval('person_id_seq');
|
||||
|
||||
Finally, you could use the OID returned from the INSERT statement to
|
||||
lookup the default value, though this is probably the least portable
|
||||
look up the default value, though this is probably the least portable
|
||||
approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the
|
||||
oid value is made available via $sth->{pg_oid_status} after
|
||||
$sth->execute().
|
||||
|
|
@ -880,8 +880,8 @@ BYTEA bytea variable-length array of bytes
|
|||
OIDs are PostgreSQL's answer to unique row ids. Every row that is
|
||||
created in PostgreSQL gets a unique OID. All OIDs generated during
|
||||
initdb are less than 16384 (from backend/access/transam.h). All
|
||||
user-created OIDs are equal or greater that this. By default, all
|
||||
these OIDs are unique not only within a table, or database, but unique
|
||||
user-created OIDs are equal to or greater than this. By default, all
|
||||
these OIDs are unique not only within a table or database, but unique
|
||||
within the entire PostgreSQL installation.
|
||||
|
||||
PostgreSQL uses OIDs in its internal system tables to link rows
|
||||
|
|
@ -965,8 +965,8 @@ BYTEA bytea variable-length array of bytes
|
|||
|
||||
4.23) Why are my subqueries using IN so slow?
|
||||
|
||||
Currently, we join subqueries to outer queries by sequential scanning
|
||||
the result of the subquery for each row of the outer query. A
|
||||
Currently, we join subqueries to outer queries by sequentially
|
||||
scanning the result of the subquery for each row of the outer query. A
|
||||
workaround is to replace IN with EXISTS:
|
||||
SELECT *
|
||||
FROM tab
|
||||
|
|
@ -1001,7 +1001,7 @@ BYTEA bytea variable-length array of bytes
|
|||
dump core?
|
||||
|
||||
The problem could be a number of things. Try testing your user-defined
|
||||
function in a stand alone test program first.
|
||||
function in a stand-alone test program first.
|
||||
|
||||
5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 not in
|
||||
alloc set!" mean?
|
||||
|
|
|
|||
|
|
@ -861,7 +861,7 @@ The row length limit will be removed in 7.1.<P>
|
|||
<H4><A NAME="4.7">4.7</A>)How much database disk space is required to
|
||||
store data from a typical text file?<BR></H4><P>
|
||||
|
||||
A PostgreSQL database may need six and a half times the disk space
|
||||
A PostgreSQL database may need six-and-a-half times the disk space
|
||||
required to store the data in a flat file.<P>
|
||||
|
||||
Consider a file of 300,000 lines with two integers on each line. The
|
||||
|
|
@ -914,7 +914,7 @@ sequential scan would be faster.<P>
|
|||
|
||||
For column-specific optimization statistics, use <SMALL>VACUUM
|
||||
ANALYZE.</SMALL> <SMALL>VACUUM ANALYZE</SMALL> is important for complex
|
||||
multi-join queries, so the optimizer can estimate the number of rows
|
||||
multijoin queries, so the optimizer can estimate the number of rows
|
||||
returned from each table, and choose the proper join order. The backend
|
||||
does not keep track of column statistics on its own, so <SMALL>VACUUM
|
||||
ANALYZE</SMALL> must be run to collect them periodically.<P>
|
||||
|
|
@ -941,20 +941,20 @@ An R-tree index is used for indexing spatial data. A hash index can't
|
|||
handle range searches. A B-tree index only handles range searches in a
|
||||
single dimension. R-tree's can handle multi-dimensional data. For
|
||||
example, if an R-tree index can be built on an attribute of type <I>point,</I>
|
||||
the system can more efficient answer queries like select all points
|
||||
within a bounding rectangle.<P>
|
||||
the system can more efficiently answer queries such as "select all points
|
||||
within a bounding rectangle."<P>
|
||||
|
||||
The canonical paper that describes the original R-Tree design is:<P>
|
||||
The canonical paper that describes the original R-tree design is:<P>
|
||||
|
||||
Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial Searching."
|
||||
Guttman, A. "R-trees: A Dynamic Index Structure for Spatial Searching."
|
||||
Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data, 45-57.<P>
|
||||
|
||||
You can also find this paper in Stonebraker's "Readings in Database
|
||||
Systems"<P>
|
||||
|
||||
Builtin R-Trees can handle polygons and boxes. In theory, R-trees can
|
||||
Built-in R-trees can handle polygons and boxes. In theory, R-trees can
|
||||
be extended to handle higher number of dimensions. In practice,
|
||||
extending R-trees require a bit of work and we don't currently have any
|
||||
extending R-trees requires a bit of work and we don't currently have any
|
||||
documentation on how to do it.<P>
|
||||
|
||||
|
||||
|
|
@ -964,13 +964,13 @@ Optimization?</H4><P>
|
|||
The GEQO module speeds query
|
||||
optimization when joining many tables by means of a Genetic
|
||||
Algorithm (GA). It allows the handling of large join queries through
|
||||
non-exhaustive search.<P>
|
||||
nonexhaustive search.<P>
|
||||
|
||||
<H4><A NAME="4.13">4.13</A>) How do I do regular expression searches and
|
||||
case-insensitive regular expression searches?</H4><P>
|
||||
|
||||
The <I>~</I> operator does regular-expression matching, and <I>~*</I>
|
||||
does case-insensitive regular-expression matching. There is no
|
||||
The <I>~</I> operator does regular expression matching, and <I>~*</I>
|
||||
does case-insensitive regular expression matching. There is no
|
||||
case-insensitive variant of the LIKE operator, but you can get the
|
||||
effect of case-insensitive <SMALL>LIKE</SMALL> with this:
|
||||
<PRE>
|
||||
|
|
@ -999,7 +999,7 @@ BYTEA bytea variable-length array of bytes
|
|||
You will see the internal name when examining system catalogs
|
||||
and in some error messages.<P>
|
||||
|
||||
The last four types above are "varlena" types (i.e. the first four bytes
|
||||
The last four types above are "varlena" types (i.e., the first four bytes
|
||||
are the length, followed by the data). <I>char(#)</I> allocates the
|
||||
maximum number of bytes no matter how much data is stored in the field.
|
||||
<I>text, varchar(#),</I> and <I>bytea</I> all have variable length on the disk,
|
||||
|
|
@ -1043,17 +1043,26 @@ One approach is to to retrieve the next SERIAL value from the sequence object wi
|
|||
$newSerialID = nextval('person_id_seq');
|
||||
INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal');
|
||||
</PRE>
|
||||
You would then also have the new value stored in <CODE>$newSerialID</CODE> for use in other queries (e.g., as a foreign key to the <CODE>person</CODE> table). Note that the name of the automatically-created SEQUENCE object will be named <<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where <I>table</I> and <I>serialcolumn</I> are the names of your table and your SERIAL column, respectively.
|
||||
|
||||
You would then also have the new value stored in
|
||||
<CODE>$newSerialID</CODE> for use in other queries (e.g., as a foreign
|
||||
key to the <CODE>person</CODE> table). Note that the name of the
|
||||
automatically created SEQUENCE object will be named
|
||||
<<I>table</I>>_<<I>serialcolumn</I>>_<I>seq</I>, where
|
||||
<I>table</I> and <I>serialcolumn</I> are the names of your table and
|
||||
your SERIAL column, respectively.
|
||||
|
||||
<P>
|
||||
Alternatively, you could retrieve the just-assigned SERIAL value with the <I>currval</I>() function <I>after</I> it was inserted by default, e.g.,
|
||||
Alternatively, you could retrieve the assigned SERIAL value with the <I>currval</I>() function <I>after</I> it was inserted by default, e.g.,
|
||||
<PRE>
|
||||
INSERT INTO person (name) VALUES ('Blaise Pascal');
|
||||
$newID = currval('person_id_seq');
|
||||
</PRE>
|
||||
Finally, you could use the <A HREF="#4.17"><small>OID</small></A> returned from the
|
||||
INSERT statement to lookup the default value, though this is probably
|
||||
the least portable approach. In Perl, using DBI with Edmund Mergl's
|
||||
DBD::Pg module, the oid value is made available via
|
||||
|
||||
Finally, you could use the <A HREF="#4.17"><small>OID</small></A>
|
||||
returned from the INSERT statement to look up the default value, though
|
||||
this is probably the least portable approach. In Perl, using DBI with
|
||||
Edmund Mergl's DBD::Pg module, the oid value is made available via
|
||||
<I>$sth->{pg_oid_status} after $sth->execute().</I>
|
||||
|
||||
<H4><A NAME="4.16.3">4.16.3</A>) Don't <I>currval()</I> and <I>nextval()</I> lead to
|
||||
|
|
@ -1065,12 +1074,13 @@ No. This is handled by the backends.
|
|||
<H4><A NAME="4.17">4.17</A>) What is an <small>OID</small>? What is a
|
||||
<small>TID</small>?</H4><P>
|
||||
|
||||
<small>OID</small>s are PostgreSQL's answer to unique row ids. Every row that is
|
||||
created in PostgreSQL gets a unique <small>OID</small>. All <small>OID</small>s generated during
|
||||
<I>initdb</I> are less than 16384 (from <I>backend/access/transam.h</I>). All
|
||||
user-created <small>OID</small>s are equal or greater that this. By default, all these
|
||||
<small>OID</small>s are unique not only within a table, or database, but unique within
|
||||
the entire PostgreSQL installation.<P>
|
||||
<small>OID</small>s are PostgreSQL's answer to unique row ids. Every
|
||||
row that is created in PostgreSQL gets a unique <small>OID</small>. All
|
||||
<small>OID</small>s generated during <I>initdb</I> are less than 16384
|
||||
(from <I>backend/access/transam.h</I>). All user-created
|
||||
<small>OID</small>s are equal to or greater than this. By default, all
|
||||
these <small>OID</small>s are unique not only within a table or
|
||||
database, but unique within the entire PostgreSQL installation.<P>
|
||||
|
||||
PostgreSQL uses <small>OID</small>s in its internal system tables to link rows between
|
||||
tables. These <small>OID</small>s can be used to identify specific user rows and used
|
||||
|
|
@ -1175,7 +1185,7 @@ Use <i>now()</i>:
|
|||
<P>
|
||||
<H4><A NAME="4.23">4.23</A>) Why are my subqueries using
|
||||
<CODE><small>IN</small></CODE> so slow?<BR></H4><P>
|
||||
Currently, we join subqueries to outer queries by sequential scanning
|
||||
Currently, we join subqueries to outer queries by sequentially scanning
|
||||
the result of the subquery for each row of the outer query. A workaround
|
||||
is to replace <CODE>IN</CODE> with <CODE>EXISTS</CODE>:
|
||||
<CODE><PRE>
|
||||
|
|
@ -1216,12 +1226,12 @@ does an <i>outer</i> join of the two tables:
|
|||
I run it in <I>psql,</I> why does it dump core?</H4><P>
|
||||
|
||||
The problem could be a number of things. Try testing your user-defined
|
||||
function in a stand alone test program first.
|
||||
function in a stand-alone test program first.
|
||||
|
||||
<H4><A NAME="5.2">5.2</A>) What does the message
|
||||
<I>"NOTICE:PortalHeapMemoryFree: 0x402251d0 not in alloc set!"</I> mean?</H4><P>
|
||||
|
||||
You are <I>pfree'ing</I> something that was not <I>palloc'ed.</I>
|
||||
You are <I>pfree'</I>ing something that was not <I>palloc'</I>ed.
|
||||
Beware of mixing <I>malloc/free</I> and <I>palloc/pfree.</I>
|
||||
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue