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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>7.8. WITH Queries (Common Table Expressions)</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="queries-values.html" title="7.7. VALUES Lists" /><link rel="next" href="datatype.html" title="Chapter 8. Data Types" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">7.8. <code xmlns="http://www.w3.org/1999/xhtml" class="literal">WITH</code> Queries (Common Table Expressions)</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="queries-values.html" title="7.7. VALUES Lists">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="queries.html" title="Chapter 7. Queries">Up</a></td><th width="60%" align="center">Chapter 7. Queries</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="datatype.html" title="Chapter 8. Data Types">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="QUERIES-WITH"><div class="titlepage"><div><div><h2 class="title" style="clear: both">7.8. <code class="literal">WITH</code> Queries (Common Table Expressions)</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="queries-with.html#QUERIES-WITH-SELECT">7.8.1. <code class="command">SELECT</code> in <code class="literal">WITH</code></a></span></dt><dt><span class="sect2"><a href="queries-with.html#QUERIES-WITH-MODIFYING">7.8.2. Data-Modifying Statements in <code class="literal">WITH</code></a></span></dt></dl></div><a id="id-1.5.6.12.2" class="indexterm"></a><a id="id-1.5.6.12.3" class="indexterm"></a><p>
   <code class="literal">WITH</code> provides a way to write auxiliary statements for use in a
   larger query.  These statements, which are often referred to as Common
   Table Expressions or <acronym class="acronym">CTE</acronym>s, can be thought of as defining
   temporary tables that exist just for one query.  Each auxiliary statement
   in a <code class="literal">WITH</code> clause can be a <code class="command">SELECT</code>,
   <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or <code class="command">DELETE</code>; and the
   <code class="literal">WITH</code> clause itself is attached to a primary statement that can
   also be a <code class="command">SELECT</code>, <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or
   <code class="command">DELETE</code>.
  </p><div class="sect2" id="QUERIES-WITH-SELECT"><div class="titlepage"><div><div><h3 class="title">7.8.1. <code class="command">SELECT</code> in <code class="literal">WITH</code></h3></div></div></div><p>
   The basic value of <code class="command">SELECT</code> in <code class="literal">WITH</code> is to
   break down complicated queries into simpler parts.  An example is:

</p><pre class="programlisting">
WITH regional_sales AS (
    SELECT region, SUM(amount) AS total_sales
    FROM orders
    GROUP BY region
), top_regions AS (
    SELECT region
    FROM regional_sales
    WHERE total_sales &gt; (SELECT SUM(total_sales)/10 FROM regional_sales)
)
SELECT region,
       product,
       SUM(quantity) AS product_units,
       SUM(amount) AS product_sales
FROM orders
WHERE region IN (SELECT region FROM top_regions)
GROUP BY region, product;
</pre><p>

   which displays per-product sales totals in only the top sales regions.
   The <code class="literal">WITH</code> clause defines two auxiliary statements named
   <code class="structname">regional_sales</code> and <code class="structname">top_regions</code>,
   where the output of <code class="structname">regional_sales</code> is used in
   <code class="structname">top_regions</code> and the output of <code class="structname">top_regions</code>
   is used in the primary <code class="command">SELECT</code> query.
   This example could have been written without <code class="literal">WITH</code>,
   but we'd have needed two levels of nested sub-<code class="command">SELECT</code>s.  It's a bit
   easier to follow this way.
  </p><p>
   <a id="id-1.5.6.12.5.3.1" class="indexterm"></a>
   The optional <code class="literal">RECURSIVE</code> modifier changes <code class="literal">WITH</code>
   from a mere syntactic convenience into a feature that accomplishes
   things not otherwise possible in standard SQL.  Using
   <code class="literal">RECURSIVE</code>, a <code class="literal">WITH</code> query can refer to its own
   output.  A very simple example is this query to sum the integers from 1
   through 100:

</p><pre class="programlisting">
WITH RECURSIVE t(n) AS (
    VALUES (1)
  UNION ALL
    SELECT n+1 FROM t WHERE n &lt; 100
)
SELECT sum(n) FROM t;
</pre><p>

   The general form of a recursive <code class="literal">WITH</code> query is always a
   <em class="firstterm">non-recursive term</em>, then <code class="literal">UNION</code> (or
   <code class="literal">UNION ALL</code>), then a
   <em class="firstterm">recursive term</em>, where only the recursive term can contain
   a reference to the query's own output.  Such a query is executed as
   follows:
  </p><div class="procedure" id="id-1.5.6.12.5.4"><p class="title"><strong>Recursive Query Evaluation</strong></p><ol class="procedure" type="1"><li class="step"><p>
     Evaluate the non-recursive term.  For <code class="literal">UNION</code> (but not
     <code class="literal">UNION ALL</code>), discard duplicate rows.  Include all remaining
     rows in the result of the recursive query, and also place them in a
     temporary <em class="firstterm">working table</em>.
    </p></li><li class="step"><p>
     So long as the working table is not empty, repeat these steps:
    </p><ol type="a" class="substeps"><li class="step"><p>
       Evaluate the recursive term, substituting the current contents of
       the working table for the recursive self-reference.
       For <code class="literal">UNION</code> (but not <code class="literal">UNION ALL</code>), discard
       duplicate rows and rows that duplicate any previous result row.
       Include all remaining rows in the result of the recursive query, and
       also place them in a temporary <em class="firstterm">intermediate table</em>.
      </p></li><li class="step"><p>
       Replace the contents of the working table with the contents of the
       intermediate table, then empty the intermediate table.
      </p></li></ol></li></ol></div><div class="note"><h3 class="title">Note</h3><p>
    Strictly speaking, this process is iteration not recursion, but
    <code class="literal">RECURSIVE</code> is the terminology chosen by the SQL standards
    committee.
   </p></div><p>
   In the example above, the working table has just a single row in each step,
   and it takes on the values from 1 through 100 in successive steps.  In
   the 100th step, there is no output because of the <code class="literal">WHERE</code>
   clause, and so the query terminates.
  </p><p>
   Recursive queries are typically used to deal with hierarchical or
   tree-structured data.  A useful example is this query to find all the
   direct and indirect sub-parts of a product, given only a table that
   shows immediate inclusions:

</p><pre class="programlisting">
WITH RECURSIVE included_parts(sub_part, part, quantity) AS (
    SELECT sub_part, part, quantity FROM parts WHERE part = 'our_product'
  UNION ALL
    SELECT p.sub_part, p.part, p.quantity
    FROM included_parts pr, parts p
    WHERE p.part = pr.sub_part
)
SELECT sub_part, SUM(quantity) as total_quantity
FROM included_parts
GROUP BY sub_part
</pre><p>
  </p><p>
   When working with recursive queries it is important to be sure that
   the recursive part of the query will eventually return no tuples,
   or else the query will loop indefinitely.  Sometimes, using
   <code class="literal">UNION</code> instead of <code class="literal">UNION ALL</code> can accomplish this
   by discarding rows that duplicate previous output rows.  However, often a
   cycle does not involve output rows that are completely duplicate: it may be
   necessary to check just one or a few fields to see if the same point has
   been reached before.  The standard method for handling such situations is
   to compute an array of the already-visited values.  For example, consider
   the following query that searches a table <code class="structname">graph</code> using a
   <code class="structfield">link</code> field:

</p><pre class="programlisting">
WITH RECURSIVE search_graph(id, link, data, depth) AS (
    SELECT g.id, g.link, g.data, 1
    FROM graph g
  UNION ALL
    SELECT g.id, g.link, g.data, sg.depth + 1
    FROM graph g, search_graph sg
    WHERE g.id = sg.link
)
SELECT * FROM search_graph;
</pre><p>

   This query will loop if the <code class="structfield">link</code> relationships contain
   cycles.  Because we require a <span class="quote">“<span class="quote">depth</span>”</span> output, just changing
   <code class="literal">UNION ALL</code> to <code class="literal">UNION</code> would not eliminate the looping.
   Instead we need to recognize whether we have reached the same row again
   while following a particular path of links.  We add two columns
   <code class="structfield">path</code> and <code class="structfield">cycle</code> to the loop-prone query:

</p><pre class="programlisting">
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (
    SELECT g.id, g.link, g.data, 1,
      ARRAY[g.id],
      false
    FROM graph g
  UNION ALL
    SELECT g.id, g.link, g.data, sg.depth + 1,
      path || g.id,
      g.id = ANY(path)
    FROM graph g, search_graph sg
    WHERE g.id = sg.link AND NOT cycle
)
SELECT * FROM search_graph;
</pre><p>

   Aside from preventing cycles, the array value is often useful in its own
   right as representing the <span class="quote">“<span class="quote">path</span>”</span> taken to reach any particular row.
  </p><p>
   In the general case where more than one field needs to be checked to
   recognize a cycle, use an array of rows.  For example, if we needed to
   compare fields <code class="structfield">f1</code> and <code class="structfield">f2</code>:

</p><pre class="programlisting">
WITH RECURSIVE search_graph(id, link, data, depth, path, cycle) AS (
    SELECT g.id, g.link, g.data, 1,
      ARRAY[ROW(g.f1, g.f2)],
      false
    FROM graph g
  UNION ALL
    SELECT g.id, g.link, g.data, sg.depth + 1,
      path || ROW(g.f1, g.f2),
      ROW(g.f1, g.f2) = ANY(path)
    FROM graph g, search_graph sg
    WHERE g.id = sg.link AND NOT cycle
)
SELECT * FROM search_graph;
</pre><p>
  </p><div class="tip"><h3 class="title">Tip</h3><p>
    Omit the <code class="literal">ROW()</code> syntax in the common case where only one field
    needs to be checked to recognize a cycle.  This allows a simple array
    rather than a composite-type array to be used, gaining efficiency.
   </p></div><div class="tip"><h3 class="title">Tip</h3><p>
    The recursive query evaluation algorithm produces its output in
    breadth-first search order.  You can display the results in depth-first
    search order by making the outer query <code class="literal">ORDER BY</code> a
    <span class="quote">“<span class="quote">path</span>”</span> column constructed in this way.
   </p></div><p>
   A helpful trick for testing queries
   when you are not certain if they might loop is to place a <code class="literal">LIMIT</code>
   in the parent query.  For example, this query would loop forever without
   the <code class="literal">LIMIT</code>:

</p><pre class="programlisting">
WITH RECURSIVE t(n) AS (
    SELECT 1
  UNION ALL
    SELECT n+1 FROM t
)
SELECT n FROM t LIMIT 100;
</pre><p>

   This works because <span class="productname">PostgreSQL</span>'s implementation
   evaluates only as many rows of a <code class="literal">WITH</code> query as are actually
   fetched by the parent query.  Using this trick in production is not
   recommended, because other systems might work differently.  Also, it
   usually won't work if you make the outer query sort the recursive query's
   results or join them to some other table, because in such cases the
   outer query will usually try to fetch all of the <code class="literal">WITH</code> query's
   output anyway.
  </p><p>
   A useful property of <code class="literal">WITH</code> queries is that they are
   normally evaluated only once per execution of the parent query, even if
   they are referred to more than once by the parent query or
   sibling <code class="literal">WITH</code> queries.
   Thus, expensive calculations that are needed in multiple places can be
   placed within a <code class="literal">WITH</code> query to avoid redundant work.  Another
   possible application is to prevent unwanted multiple evaluations of
   functions with side-effects.
   However, the other side of this coin is that the optimizer is not able to
   push restrictions from the parent query down into a multiply-referenced
   <code class="literal">WITH</code> query, since that might affect all uses of the
   <code class="literal">WITH</code> query's output when it should affect only one.
   The multiply-referenced <code class="literal">WITH</code> query will be
   evaluated as written, without suppression of rows that the parent query
   might discard afterwards.  (But, as mentioned above, evaluation might stop
   early if the reference(s) to the query demand only a limited number of
   rows.)
  </p><p>
   However, if a <code class="literal">WITH</code> query is non-recursive and
   side-effect-free (that is, it is a <code class="literal">SELECT</code> containing
   no volatile functions) then it can be folded into the parent query,
   allowing joint optimization of the two query levels.  By default, this
   happens if the parent query references the <code class="literal">WITH</code> query
   just once, but not if it references the <code class="literal">WITH</code> query
   more than once.  You can override that decision by
   specifying <code class="literal">MATERIALIZED</code> to force separate calculation
   of the <code class="literal">WITH</code> query, or by specifying <code class="literal">NOT
   MATERIALIZED</code> to force it to be merged into the parent query.
   The latter choice risks duplicate computation of
   the <code class="literal">WITH</code> query, but it can still give a net savings if
   each usage of the <code class="literal">WITH</code> query needs only a small part
   of the <code class="literal">WITH</code> query's full output.
  </p><p>
   A simple example of these rules is
</p><pre class="programlisting">
WITH w AS (
    SELECT * FROM big_table
)
SELECT * FROM w WHERE key = 123;
</pre><p>
   This <code class="literal">WITH</code> query will be folded, producing the same
   execution plan as
</p><pre class="programlisting">
SELECT * FROM big_table WHERE key = 123;
</pre><p>
   In particular, if there's an index on <code class="structfield">key</code>,
   it will probably be used to fetch just the rows having <code class="literal">key =
   123</code>.  On the other hand, in
</p><pre class="programlisting">
WITH w AS (
    SELECT * FROM big_table
)
SELECT * FROM w AS w1 JOIN w AS w2 ON w1.key = w2.ref
WHERE w2.key = 123;
</pre><p>
   the <code class="literal">WITH</code> query will be materialized, producing a
   temporary copy of <code class="structname">big_table</code> that is then
   joined with itself — without benefit of any index.  This query
   will be executed much more efficiently if written as
</p><pre class="programlisting">
WITH w AS NOT MATERIALIZED (
    SELECT * FROM big_table
)
SELECT * FROM w AS w1 JOIN w AS w2 ON w1.key = w2.ref
WHERE w2.key = 123;
</pre><p>
   so that the parent query's restrictions can be applied directly
   to scans of <code class="structname">big_table</code>.
  </p><p>
   An example where <code class="literal">NOT MATERIALIZED</code> could be
   undesirable is
</p><pre class="programlisting">
WITH w AS (
    SELECT key, very_expensive_function(val) as f FROM some_table
)
SELECT * FROM w AS w1 JOIN w AS w2 ON w1.f = w2.f;
</pre><p>
   Here, materialization of the <code class="literal">WITH</code> query ensures
   that <code class="function">very_expensive_function</code> is evaluated only
   once per table row, not twice.
  </p><p>
   The examples above only show <code class="literal">WITH</code> being used with
   <code class="command">SELECT</code>, but it can be attached in the same way to
   <code class="command">INSERT</code>, <code class="command">UPDATE</code>, or <code class="command">DELETE</code>.
   In each case it effectively provides temporary table(s) that can
   be referred to in the main command.
  </p></div><div class="sect2" id="QUERIES-WITH-MODIFYING"><div class="titlepage"><div><div><h3 class="title">7.8.2. Data-Modifying Statements in <code class="literal">WITH</code></h3></div></div></div><p>
    You can use data-modifying statements (<code class="command">INSERT</code>,
    <code class="command">UPDATE</code>, or <code class="command">DELETE</code>) in <code class="literal">WITH</code>.  This
    allows you to perform several different operations in the same query.
    An example is:

</p><pre class="programlisting">
WITH moved_rows AS (
    DELETE FROM products
    WHERE
        "date" &gt;= '2010-10-01' AND
        "date" &lt; '2010-11-01'
    RETURNING *
)
INSERT INTO products_log
SELECT * FROM moved_rows;
</pre><p>

    This query effectively moves rows from <code class="structname">products</code> to
    <code class="structname">products_log</code>.  The <code class="command">DELETE</code> in <code class="literal">WITH</code>
    deletes the specified rows from <code class="structname">products</code>, returning their
    contents by means of its <code class="literal">RETURNING</code> clause; and then the
    primary query reads that output and inserts it into
    <code class="structname">products_log</code>.
   </p><p>
    A fine point of the above example is that the <code class="literal">WITH</code> clause is
    attached to the <code class="command">INSERT</code>, not the sub-<code class="command">SELECT</code> within
    the <code class="command">INSERT</code>.  This is necessary because data-modifying
    statements are only allowed in <code class="literal">WITH</code> clauses that are attached
    to the top-level statement.  However, normal <code class="literal">WITH</code> visibility
    rules apply, so it is possible to refer to the <code class="literal">WITH</code>
    statement's output from the sub-<code class="command">SELECT</code>.
   </p><p>
    Data-modifying statements in <code class="literal">WITH</code> usually have
    <code class="literal">RETURNING</code> clauses (see <a class="xref" href="dml-returning.html" title="6.4. Returning Data From Modified Rows">Section 6.4</a>),
    as shown in the example above.
    It is the output of the <code class="literal">RETURNING</code> clause, <span class="emphasis"><em>not</em></span> the
    target table of the data-modifying statement, that forms the temporary
    table that can be referred to by the rest of the query.  If a
    data-modifying statement in <code class="literal">WITH</code> lacks a <code class="literal">RETURNING</code>
    clause, then it forms no temporary table and cannot be referred to in
    the rest of the query.  Such a statement will be executed nonetheless.
    A not-particularly-useful example is:

</p><pre class="programlisting">
WITH t AS (
    DELETE FROM foo
)
DELETE FROM bar;
</pre><p>

    This example would remove all rows from tables <code class="structname">foo</code> and
    <code class="structname">bar</code>.  The number of affected rows reported to the client
    would only include rows removed from <code class="structname">bar</code>.
   </p><p>
    Recursive self-references in data-modifying statements are not
    allowed.  In some cases it is possible to work around this limitation by
    referring to the output of a recursive <code class="literal">WITH</code>, for example:

</p><pre class="programlisting">
WITH RECURSIVE included_parts(sub_part, part) AS (
    SELECT sub_part, part FROM parts WHERE part = 'our_product'
  UNION ALL
    SELECT p.sub_part, p.part
    FROM included_parts pr, parts p
    WHERE p.part = pr.sub_part
)
DELETE FROM parts
  WHERE part IN (SELECT part FROM included_parts);
</pre><p>

    This query would remove all direct and indirect subparts of a product.
   </p><p>
    Data-modifying statements in <code class="literal">WITH</code> are executed exactly once,
    and always to completion, independently of whether the primary query
    reads all (or indeed any) of their output.  Notice that this is different
    from the rule for <code class="command">SELECT</code> in <code class="literal">WITH</code>: as stated in the
    previous section, execution of a <code class="command">SELECT</code> is carried only as far
    as the primary query demands its output.
   </p><p>
    The sub-statements in <code class="literal">WITH</code> are executed concurrently with
    each other and with the main query.  Therefore, when using data-modifying
    statements in <code class="literal">WITH</code>, the order in which the specified updates
    actually happen is unpredictable.  All the statements are executed with
    the same <em class="firstterm">snapshot</em> (see <a class="xref" href="mvcc.html" title="Chapter 13. Concurrency Control">Chapter 13</a>), so they
    cannot <span class="quote">“<span class="quote">see</span>”</span> one another's effects on the target tables.  This
    alleviates the effects of the unpredictability of the actual order of row
    updates, and means that <code class="literal">RETURNING</code> data is the only way to
    communicate changes between different <code class="literal">WITH</code> sub-statements and
    the main query.  An example of this is that in

</p><pre class="programlisting">
WITH t AS (
    UPDATE products SET price = price * 1.05
    RETURNING *
)
SELECT * FROM products;
</pre><p>

    the outer <code class="command">SELECT</code> would return the original prices before the
    action of the <code class="command">UPDATE</code>, while in

</p><pre class="programlisting">
WITH t AS (
    UPDATE products SET price = price * 1.05
    RETURNING *
)
SELECT * FROM t;
</pre><p>

    the outer <code class="command">SELECT</code> would return the updated data.
   </p><p>
    Trying to update the same row twice in a single statement is not
    supported.  Only one of the modifications takes place, but it is not easy
    (and sometimes not possible) to reliably predict which one.  This also
    applies to deleting a row that was already updated in the same statement:
    only the update is performed.  Therefore you should generally avoid trying
    to modify a single row twice in a single statement.  In particular avoid
    writing <code class="literal">WITH</code> sub-statements that could affect the same rows
    changed by the main statement or a sibling sub-statement.  The effects
    of such a statement will not be predictable.
   </p><p>
    At present, any table used as the target of a data-modifying statement in
    <code class="literal">WITH</code> must not have a conditional rule, nor an <code class="literal">ALSO</code>
    rule, nor an <code class="literal">INSTEAD</code> rule that expands to multiple statements.
   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="queries-values.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="queries.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="datatype.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">7.7. <code class="literal">VALUES</code> Lists </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 8. Data Types</td></tr></table></div></body></html>