</para>

<para>

The executor mechanism is used to evaluate all four basic SQL query types:

<command>SELECT</>, <command>INSERT</>, <command>UPDATE</>, and

<command>DELETE</>. For <command>SELECT</>, the top-level executor

code only needs to send each row returned by the query plan tree off

to the client. For <command>INSERT</>, each returned row is inserted

into the target table specified for the <command>INSERT</>. This is

done in a special top-level plan node called <literal>ModifyTable</>.

(A simple

<command>INSERT ... VALUES</> command creates a trivial plan tree

consisting of a single <literal>Result</> node, which computes just one

result row, and <literal>ModifyTable</> above it to perform the insertion.

But <command>INSERT ... SELECT</> can demand the full power

of the executor mechanism.) For <command>UPDATE</>, the planner arranges

that each computed row includes all the updated column values, plus

the <firstterm>TID</> (tuple ID, or row ID) of the original target row;

this data is fed into a <literal>ModifyTable</> node, which uses the

information to create a new updated row and mark the old row deleted.

For <command>DELETE</>, the only column that is actually returned by the

plan is the TID, and the <literal>ModifyTable</> 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>