Postgres 13

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CREATE OR REPLACE FUNCTION public.truncate_if_exists(_table text, _schema text DEFAULT NULL)
  RETURNS text
  LANGUAGE plpgsql AS
$func$
DECLARE
   _qual_tbl text := concat_ws('.', quote_ident(_schema), quote_ident(_table));
   _row_found bool;
BEGIN
   IF to_regclass(_qual_tbl) IS NOT NULL THEN   -- table exists
      EXECUTE 'SELECT EXISTS (SELECT FROM ' || _qual_tbl || ')'
      INTO _row_found;

      IF _row_found THEN                        -- table is not empty
         EXECUTE 'TRUNCATE ' || _qual_tbl;
         RETURN 'Table truncated: ' || _qual_tbl;
      ELSE  -- optional!
         RETURN 'Table exists but is empty: ' || _qual_tbl;
      END IF;
   ELSE  -- optional!
      RETURN 'Table not found: ' || _qual_tbl;
   END IF;
END
$func$;

CREATE FUNCTION

-- default schema
CREATE TABLE "CaMeL" (id int);
INSERT INTO "CaMeL" VALUES (1);

CREATE TABLE tbl_full (id int);
INSERT INTO tbl_full VALUES (1);

CREATE TABLE tbl_empty (id int);  --  no rows

-- temp schema
CREATE TEMP TABLE tmp_tbl (id int);  -- temporary table
INSERT INTO tmp_tbl VALUES (1);

CREATE TABLE

INSERT 0 1

CREATE TABLE

INSERT 0 1

CREATE TABLE

CREATE TABLE

INSERT 0 1

-- custom schema, correct capitalizsation!
SELECT public.truncate_if_exists('CaMeL', 'custom_schema');

truncate_if_exists

Table not found: custom_schema."CaMeL"

SELECT 1

-- default schema, correct capitalizsation!
SELECT public.truncate_if_exists('CaMeL');

truncate_if_exists

Table truncated: "CaMeL"

SELECT 1

-- now, table is empty now:
SELECT public.truncate_if_exists('CaMeL');

truncate_if_exists

Table exists but is empty: "CaMeL"

SELECT 1

-- plain table with rows
SELECT public.truncate_if_exists('tbl_full');

truncate_if_exists

Table truncated: tbl_full

SELECT 1

-- plain table without rows
SELECT public.truncate_if_exists('tbl_empty');

truncate_if_exists

Table exists but is empty: tbl_empty

SELECT 1

-- temp table
SELECT public.truncate_if_exists('tmp_tbl');

truncate_if_exists

Table truncated: tmp_tbl

SELECT 1

-- temp table, the safe way:
SELECT public.truncate_if_exists('tmp_tbl', pg_my_temp_schema()::regnamespace::text);

truncate_if_exists

Table exists but is empty: pg_temp_3.tmp_tbl

SELECT 1