Dynamic Commands in PL/pgSQL

The execute keyword allows you to run a dynamic command - where perhaps a table or column name is not known ahead of time.

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create or replace function get_row_count(table_name text)
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  returns integer
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  as $$
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declare
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  c int;
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begin
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  execute format('select count(*) from %I', table_name) into c;
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  return c;
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end;
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$$ language plpgsql;
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create table s (id serial primary key);
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insert into s default values;
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create table t (id serial primary key);
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insert into t default values;
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insert into t default values;
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select get_row_count('s'); -- 1
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select get_row_count('t'); -- 2

Some notes from the docs:

There is no plan caching for dynamic commands - commands executed via execute.
Be careful not to do something like key = null, where null is passed in dynamically. (Use is distinct from).
See the using clause for passing parameters to the dynamic command.

You can read more about this here.