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SEARCH and CYCLE clauses

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This adds the SQL standard feature that adds the SEARCH and CYCLE
clauses to recursive queries to be able to do produce breadth- or
depth-first search orders and detect cycles.  These clauses can be
rewritten into queries using existing syntax, and that is what this
patch does in the rewriter.

Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/db80ceee-6f97-9b4a-8ee8-3ba0c58e5be2@2ndquadrant.com
This commit is contained in:
Peter Eisentraut
2021-02-01 13:54:59 +01:00
parent bb513b364b
commit 3696a600e2
28 changed files with 2301 additions and 33 deletions
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44
doc/src/sgml/ref/select.sgml
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@@ -73,6 +73,8 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac
<phrase>and <replaceable class="parameter">with_query</replaceable> is:</phrase>
<replaceable class="parameter">with_query_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ] AS [ [ NOT ] MATERIALIZED ] ( <replaceable class="parameter">select</replaceable> | <replaceable class="parameter">values</replaceable> | <replaceable class="parameter">insert</replaceable> | <replaceable class="parameter">update</replaceable> | <replaceable class="parameter">delete</replaceable> )
[ SEARCH { BREADTH | DEPTH } FIRST BY <replaceable>column_name</replaceable> [, ...] SET <replaceable>search_seq_col_name</replaceable> ]
[ CYCLE <replaceable>column_name</replaceable> [, ...] SET <replaceable>cycle_mark_col_name</replaceable> TO <replaceable>cycle_mark_value</replaceable> DEFAULT <replaceable>cycle_mark_default</replaceable> USING <replaceable>cycle_path_col_name</replaceable> ]
TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
</synopsis>
@@ -276,6 +278,48 @@ TABLE [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ]
queries that do not use recursion or forward references.
</para>
<para>
The optional <literal>SEARCH</literal> clause computes a <firstterm>search
sequence column</firstterm> that can be used for ordering the results of a
recursive query in either breadth-first or depth-first order. The
supplied column name list specifies the row key that is to be used for
keeping track of visited rows. A column named
<replaceable>search_seq_col_name</replaceable> will be added to the result
column list of the <literal>WITH</literal> query. This column can be
ordered by in the outer query to achieve the respective ordering. See
<xref linkend="queries-with-search"/> for examples.
</para>
<para>
The optional <literal>CYCLE</literal> clause is used to detect cycles in
recursive queries. The supplied column name list specifies the row key
that is to be used for keeping track of visited rows. A column named
<replaceable>cycle_mark_col_name</replaceable> will be added to the result
column list of the <literal>WITH</literal> query. This column will be set
to <replaceable>cycle_mark_value</replaceable> when a cycle has been
detected, else to <replaceable>cycle_mark_default</replaceable>.
Furthermore, processing of the recursive union will stop when a cycle has
been detected. <replaceable>cycle_mark_value</replaceable> and
<replaceable>cycle_mark_default</replaceable> must be constants and they
must be coercible to a common data type, and the data type must have an
inequality operator. (The SQL standard requires that they be character
strings, but PostgreSQL does not require that.) Furthermore, a column
named <replaceable>cycle_path_col_name</replaceable> will be added to the
result column list of the <literal>WITH</literal> query. This column is
used internally for tracking visited rows. See <xref
linkend="queries-with-cycle"/> for examples.
</para>
<para>
Both the <literal>SEARCH</literal> and the <literal>CYCLE</literal> clause
are only valid for recursive <literal>WITH</literal> queries. The
<replaceable>with_query</replaceable> must be a <literal>UNION</literal>
(or <literal>UNION ALL</literal>) of two <literal>SELECT</literal> (or
equivalent) commands (no nested <literal>UNION</literal>s). If both
clauses are used, the column added by the <literal>SEARCH</literal> clause
appears before the columns added by the <literal>CYCLE</literal> clause.
</para>
<para>
The primary query and the <literal>WITH</literal> queries are all
(notionally) executed at the same time. This implies that the effects of