Slow query with where on joined column - php

Question

I tried to combine 2 queries into 1 by using JOINs
// Query 1
$entity = \DB::select("
SELECT memo_id
FROM entities
WHERE entity_type = 4
AND entity_id = '".$entity_nr."'
LIMIT 1
");
$memo = \DB::select("
SELECT memo
FROM memos
WHERE id = '".$entity[0]->memo_id."'
LIMIT 1
");
// Query 2
$memo = \DB::select('
SELECT memo
FROM memos
JOIN entities ON "memos"."id" = "entities"."memo_id"
WHERE "entities"."entity_type" = 4
AND "entities"."entity_id" IN (?)
LIMIT 1
', [$entity_nr]);
The 2 query's of Query 1 are done in less than a second. Query 2 takes several seconds. If i remove the where clause of Query 2 it executes fast. Using AND "entities"."entity_id" = ? did'nt helped as well.
Howto solve this?
UPDATE 2020-09-27
Refactoring my question. Hopes this makes it clearer. Expected output is to get the memo. There is always only 1 possible match. In the example code im not checking for existence of the result, because it doesn't really add something to the question.
Database which im using is "Actian Zen database".
Table structure
//////////////////////////
// Table: "entity_memo" //
//////////////////////////
CREATE TABLE "entity_memo" (
"entity_type" SMALLINT,
"entity_id" CHAR(15),
"memo_id" INTEGER
);
CREATE UNIQUE INDEX "key0" ON "entity_memo" ( "entity_type", "entity_id", "memo_id" );
CREATE INDEX "key1" ON "entity_memo" ( "memo_id", "entity_type", "entity_id" );
//////////////////////////
// Table: "memos" //
//////////////////////////
CREATE TABLE "memos" (
"id" INTEGER,
"memo" LVAR(32002)
);
CREATE UNIQUE INDEX "key0" ON "memos" ( "id" );
Option 1 (= fast, but 2 queries)
$entity_type = 1;
$entity_id = 'ABC123456';
$entity = \DB::select('
SELECT memo_id
FROM entity_memo
WHERE entity_type = ?
AND entity_id = ?
LIMIT 1
', [$entity_type, $entity_id]
);
$memo = \DB::select('
SELECT memo
FROM memos
WHERE id = ?
LIMIT 1
', [$entity[0]->memo_id]
);
return $memo[0]->memo;
Option 2 (= slow, but 1 query only)
$entity_type = 1;
$entity_id = 'ABC123456';
$memo = \DB::select('
SELECT memo
FROM memos
JOIN entity_memo ON "memos"."id" = "entity_memo"."memo_id"
WHERE "entity_memo"."entity_type" = ?
AND "entity_memo"."entity_id" = ?
LIMIT 1
', [$entity_type, $entity_id]
);
return $memo[0]->memo;

Answer 1

It will be look simpler, if you write it without JOINs.
SELECT ent.unique_id, mem.memo
FROM entities ent, memos mem
WHERE ent.unique_id = mem.unique_id
AND ent.entity_type = 4
AND ent.entity_id = '".$entity_nr."'
But the cause of query working slow is in tables itself. You should to add indexes to PK and FK columns for query to work faster.