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| -rw-r--r-- | doc/src/sgml/arch-dev.sgml | 50 |
1 files changed, 30 insertions, 20 deletions
diff --git a/doc/src/sgml/arch-dev.sgml b/doc/src/sgml/arch-dev.sgml index 9ffb8427bf0..fb8a8bde009 100644 --- a/doc/src/sgml/arch-dev.sgml +++ b/doc/src/sgml/arch-dev.sgml @@ -528,26 +528,36 @@ </para> <para> - The executor mechanism is used to evaluate all four basic SQL query types: - <command>SELECT</command>, <command>INSERT</command>, <command>UPDATE</command>, and - <command>DELETE</command>. For <command>SELECT</command>, the top-level executor - code only needs to send each row returned by the query plan tree off - to the client. For <command>INSERT</command>, each returned row is inserted - into the target table specified for the <command>INSERT</command>. This is - done in a special top-level plan node called <literal>ModifyTable</literal>. - (A simple - <command>INSERT ... VALUES</command> command creates a trivial plan tree - consisting of a single <literal>Result</literal> node, which computes just one - result row, and <literal>ModifyTable</literal> above it to perform the insertion. - But <command>INSERT ... SELECT</command> can demand the full power - of the executor mechanism.) For <command>UPDATE</command>, the planner arranges - that each computed row includes all the updated column values, plus - the <firstterm>TID</firstterm> (tuple ID, or row ID) of the original target row; - this data is fed into a <literal>ModifyTable</literal> node, which uses the - information to create a new updated row and mark the old row deleted. - For <command>DELETE</command>, the only column that is actually returned by the - plan is the TID, and the <literal>ModifyTable</literal> node simply uses the TID - to visit each target row and mark it deleted. + The executor mechanism is used to evaluate all four basic SQL query + types: <command>SELECT</command>, <command>INSERT</command>, + <command>UPDATE</command>, and <command>DELETE</command>. + For <command>SELECT</command>, the top-level executor code + only needs to send each row returned by the query plan tree + off to the client. <command>INSERT ... SELECT</command>, + <command>UPDATE</command>, and <command>DELETE</command> + are effectively <command>SELECT</command>s under a special + top-level plan node called <literal>ModifyTable</literal>. + </para> + + <para> + <command>INSERT ... SELECT</command> feeds the rows up + to <literal>ModifyTable</literal> for insertion. For + <command>UPDATE</command>, the planner arranges that each + computed row includes all the updated column values, plus the + <firstterm>TID</firstterm> (tuple ID, or row ID) of the original + target row; this data is fed up to the <literal>ModifyTable</literal> + node, which uses the information to create a new updated row and + mark the old row deleted. For <command>DELETE</command>, the only + column that is actually returned by the plan is the TID, and the + <literal>ModifyTable</literal> node simply uses the TID to visit each + target row and mark it deleted. + </para> + + <para> + A simple <command>INSERT ... VALUES</command> command creates a + trivial plan tree consisting of a single <literal>Result</literal> + node, which computes just one result row, feeding that up + to<literal>ModifyTable</literal> to perform the insertion. </para> </sect1> |