database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-28 23:42:10 +03:00
Code Activity

This patch implements holdable cursors, following the proposal

Browse Source 
(materialization into a tuple store) discussed on pgsql-hackers earlier.
I've updated the documentation and the regression tests.

Notes on the implementation:

- I needed to change the tuple store API slightly -- it assumes that it
won't be used to hold data across transaction boundaries, so the temp
files that it uses for on-disk storage are automatically reclaimed at
end-of-transaction. I added a flag to tuplestore_begin_heap() to control
this behavior. Is changing the tuple store API in this fashion OK?

- in order to store executor results in a tuple store, I added a new
CommandDest. This works well for the most part, with one exception: the
current DestFunction API doesn't provide enough information to allow the
Executor to store results into an arbitrary tuple store (where the
particular tuple store to use is chosen by the call site of
ExecutorRun). To workaround this, I've temporarily hacked up a solution
that works, but is not ideal: since the receiveTuple DestFunction is
passed the portal name, we can use that to lookup the Portal data
structure for the cursor and then use that to get at the tuple store the
Portal is using. This unnecessarily ties the Portal code with the
tupleReceiver code, but it works...

The proper fix for this is probably to change the DestFunction API --
Tom suggested passing the full QueryDesc to the receiveTuple function.
In that case, callers of ExecutorRun could "subclass" QueryDesc to add
any additional fields that their particular CommandDest needed to get
access to. This approach would work, but I'd like to think about it for
a little bit longer before deciding which route to go. In the mean time,
the code works fine, so I don't think a fix is urgent.

- (semi-related) I added a NO SCROLL keyword to DECLARE CURSOR, and
adjusted the behavior of SCROLL in accordance with the discussion on
-hackers.

- (unrelated) Cleaned up some SGML markup in sql.sgml, copy.sgml

Neil Conway
This commit is contained in:
Bruce Momjian
2003-03-27 16:51:29 +00:00
parent 7638087882
commit 54f7338fa1
35 changed files with 850 additions and 314 deletions
Download Patch File Download Diff File Expand all files Collapse all files

106
doc/src/sgml/sql.sgml
View File

@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.29 2003/02/19 04:06:28 momjian Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/sql.sgml,v 1.30 2003/03/27 16:51:26 momjian Exp $
-->
<chapter id="sql">
@ -851,7 +851,7 @@ A &lt; B + 3.
<para>
The most often used command in <acronym>SQL</acronym> is the
SELECT statement,
<command>SELECT</command> statement,
used to retrieve data. The syntax is:
<synopsis>
@ -881,7 +881,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
<title>Simple Selects</title>
<para>
Here are some simple examples using a SELECT statement:
Here are some simple examples using a <command>SELECT</command> statement:
<example>
<title id="simple-query">Simple Query with Qualification</title>
@ -905,9 +905,10 @@ SELECT * FROM PART
</para>
<para>
Using <quote>*</quote> in the SELECT statement will deliver all attributes from
the table. If we want to retrieve only the attributes PNAME and PRICE
from table PART we use the statement:
Using <quote>*</quote> in the <command>SELECT</command> statement
will deliver all attributes from the table. If we want to retrieve
only the attributes PNAME and PRICE from table PART we use the
statement:
<programlisting>
SELECT PNAME, PRICE
@ -924,9 +925,9 @@ SELECT PNAME, PRICE
Cam | 25
</programlisting>
Note that the <acronym>SQL</acronym> SELECT corresponds to the
<quote>projection</quote> in relational algebra not to the
<quote>selection</quote> (see <xref linkend="rel-alg"
Note that the <acronym>SQL</acronym> <command>SELECT</command>
corresponds to the <quote>projection</quote> in relational algebra
not to the <quote>selection</quote> (see <xref linkend="rel-alg"
endterm="rel-alg"> for more details).
</para>
@ -1252,15 +1253,15 @@ select sname, pname from supplier
<title id="aggregates-tutorial">Aggregate Operators</title>
<para>
<acronym>SQL</acronym> provides aggregate operators
(e.g. AVG, COUNT, SUM, MIN, MAX) that
take an expression as argument. The expression is evaluated at
each row that satisfies the WHERE clause, and the aggregate operator
is calculated over this set of input values. Normally, an aggregate
delivers a single result for a whole SELECT statement. But if
grouping is specified in the query, then a separate calculation is done
over the rows of each group, and an aggregate result is delivered per
group (see next section).
<acronym>SQL</acronym> provides aggregate operators (e.g. AVG,
COUNT, SUM, MIN, MAX) that take an expression as argument. The
expression is evaluated at each row that satisfies the WHERE
clause, and the aggregate operator is calculated over this set
of input values. Normally, an aggregate delivers a single
result for a whole <command>SELECT</command> statement. But if
grouping is specified in the query, then a separate calculation
is done over the rows of each group, and an aggregate result is
delivered per group (see next section).
<example>
<title id="aggregates-example">Aggregates</title>
@ -1413,11 +1414,12 @@ SELECT S.SNO, S.SNAME, COUNT(SE.PNO)
</para>
<para>
Also observe that it makes no sense to ask for an aggregate of an
aggregate, e.g., AVG(MAX(sno)), because a SELECT only does one pass
of grouping and aggregation. You can get a result of this kind by
using a temporary table or a sub-SELECT in the FROM clause to
do the first level of aggregation.
Also observe that it makes no sense to ask for an aggregate of
an aggregate, e.g., AVG(MAX(sno)), because a
<command>SELECT</command> only does one pass of grouping and
aggregation. You can get a result of this kind by using a
temporary table or a sub-SELECT in the FROM clause to do the
first level of aggregation.
</para>
</sect3>
@ -1502,16 +1504,18 @@ SELECT *
</para>
<para>
When we look at the above query we can see
the keyword SELECT two times. The first one at the beginning of the
query - we will refer to it as outer SELECT - and the one in the WHERE
clause which begins a nested query - we will refer to it as inner
SELECT. For every tuple of the outer SELECT the inner SELECT has to be
evaluated. After every evaluation we know the price of the tuple named
'Screw' and we can check if the price of the actual tuple is
greater. (Actually, in this example the inner query need only be
evaluated once, since it does not depend on the state of the outer
query.)
When we look at the above query we can see the keyword
<command>SELECT</command> two times. The first one at the
beginning of the query - we will refer to it as outer
<command>SELECT</command> - and the one in the WHERE clause which
begins a nested query - we will refer to it as inner
<command>SELECT</command>. For every tuple of the outer
<command>SELECT</command> the inner <command>SELECT</command> has
to be evaluated. After every evaluation we know the price of the
tuple named 'Screw' and we can check if the price of the actual
tuple is greater. (Actually, in this example the inner query need
only be evaluated once, since it does not depend on the state of
the outer query.)
</para>
<para>
@ -1528,11 +1532,13 @@ SELECT *
</para>
<para>
In our example the result will be empty because every supplier sells
at least one part. Note that we use S.SNO from the outer SELECT within
the WHERE clause of the inner SELECT. Here the subquery must be
evaluated afresh for each tuple from the outer query, i.e. the value for
S.SNO is always taken from the current tuple of the outer SELECT.
In our example the result will be empty because every supplier
sells at least one part. Note that we use S.SNO from the outer
<command>SELECT</command> within the WHERE clause of the inner
<command>SELECT</command>. Here the subquery must be evaluated
afresh for each tuple from the outer query, i.e. the value for
S.SNO is always taken from the current tuple of the outer
<command>SELECT</command>.
</para>
</example>
</para>
@ -1670,7 +1676,7 @@ EXCEPT
<para>
The most fundamental command for data definition is the
one that creates a new relation (a new table). The syntax of the
CREATE TABLE command is:
<command>CREATE TABLE</command> command is:
<synopsis>
CREATE TABLE <replaceable class="parameter">table_name</replaceable>
@ -1786,7 +1792,7 @@ CREATE TABLE SELLS
<para>
To create an index in <acronym>SQL</acronym>
the CREATE INDEX command is used. The syntax is:
the <command>CREATE INDEX</command> command is used. The syntax is:
<programlisting>
CREATE INDEX <replaceable class="parameter">index_name</replaceable>
@ -1808,10 +1814,11 @@ CREATE INDEX I ON SUPPLIER (SNAME);
</para>
<para>
The created index is maintained automatically, i.e. whenever a new tuple
is inserted into the relation SUPPLIER the index I is adapted. Note
that the only changes a user can perceive when an index is present
are increased speed for SELECT and decreases in speed of updates.
The created index is maintained automatically, i.e. whenever a new
tuple is inserted into the relation SUPPLIER the index I is
adapted. Note that the only changes a user can perceive when an
index is present are increased speed for <command>SELECT</command>
and decreases in speed of updates.
</para>
</example>
</para>
@ -1916,7 +1923,7 @@ SELECT * FROM London_Suppliers
<para>
To destroy a table (including all tuples stored in that table) the
DROP TABLE command is used:
<command>DROP TABLE</command> command is used:
<programlisting>
DROP TABLE <replaceable class="parameter">table_name</replaceable>;
@ -1932,7 +1939,7 @@ DROP TABLE SUPPLIER;
</para>
<para>
The DROP INDEX command is used to destroy an index:
The <command>DROP INDEX</command> command is used to destroy an index:
<programlisting>
DROP INDEX <replaceable class="parameter">index_name</replaceable>;
@ -1940,7 +1947,8 @@ DROP INDEX <replaceable class="parameter">index_name</replaceable>;
</para>
<para>
Finally to destroy a given view use the command DROP VIEW:
Finally to destroy a given view use the command <command>DROP
VIEW</command>:
<programlisting>
DROP VIEW <replaceable class="parameter">view_name</replaceable>;
@ -1994,7 +2002,7 @@ INSERT INTO SELLS (SNO, PNO)
<para>
To change one or more attribute values of tuples in a relation the
UPDATE command is used. The syntax is:
<command>UPDATE</command> command is used. The syntax is:
<programlisting>
UPDATE <replaceable class="parameter">table_name</replaceable>
@ -2126,7 +2134,7 @@ DELETE FROM SUPPLIER
need a mechanism to access every single tuple of the set of tuples
returned by a SELECT statement. This mechanism can be provided by
declaring a <firstterm>cursor</firstterm>.
After that we can use the FETCH command to
After that we can use the <command>FETCH</command> command to
retrieve a tuple and set the cursor to the next tuple.
</para>