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.