Unable to FORCE INDEX on LEFT JOIN subquery? - mysql

Question

I have a complicated query which involves a subquery inside a LEFT JOIN statement. The process time is ~2s.
If I replace the subquery with a temporary table, I hit the same ~2s.
If I create an index in the temporary table, I have ~0.05s.
I am trying to figure out why I can't work with Mysql USE INDEX or FORCE INDEX directly in the LEFT JOIN STATEMENT.
Here is a simple query which will return a 'check your mysql syntax' error.
SELECT *
FROM table1 t1
LEFT JOIN (SELECT id_project FROM table2) t2
FORCE INDEX FOR JOIN (id_project)
ON (t2.id_project = t1.id_project);
Why can't I use a FORCE INDEX with a LEFT JOIN subquery?
--edit: here is the actual query returning mysql error
SELECT
p.id_projet, pa.piste_affaire, ty.type, p.e_force, gr.groupe, p.client, p.intitule_affaire,
ag.agence, st.statut, p.winratio, p.justification_nogo, p.webdoc, sp.sponsor, dis.domain_is,
cons.constructeur, p.date_chgt_statut, p.date_creation,
p.ca_estimatif, p.tcv, p.marge,
tarp.target_price, p.marketable_price,
p.remise_propal_initiale, p.remise_propal_reel,
p.date_de_signature, p.debut_build, p.fin_build,
his.date, his.login, co.contrat,
p.duree_contrat, p.fqp, cap.capitalisation,
p.international, p.risk_management, p.asi,
GROUP_CONCAT(com.commentaire,',') AS COMMENT
FROM
piste_affaire pa, statut st, TYPE ty, agence ag,
domain_is dis, sponsor sp, constructeur cons, groupe gr,
target_price tarp, contrat co, historique his, capitalisation cap,
projet p
LEFT JOIN commentaire com ON (p.id_projet=com.id_projet)
LEFT JOIN bizdev bizd ON (p.id_bizdev = bizd.id_bizdev )
LEFT JOIN (SELECT eco_projet.id_projet,
GROUP_CONCAT(
CONCAT(ecosysteme)
ORDER BY ecosysteme SEPARATOR ', ' ) AS ecosysteme
FROM ecosysteme NATURAL JOIN eco_projet, projet
WHERE eco_projet.id_projet = projet.id_projet
GROUP BY eco_projet.id_projet) AS tmpeco
/* if I delete that line, everything works as intended with ~2s query*/
FORCE INDEX FOR JOIN (id_projet)
/* ** */
ON (tmpeco.id_projet = p.id_projet)
WHERE
ag.id_division != 2 AND
dis.id_division != 2 AND
st.id_division != 2 AND
pa.id_division != 2 AND
cons.id_division != 2 AND
p.type_division = 1 AND
pa.id_piste_affaire = p.id_piste_affaire AND
p.id_statut = st.id_statut AND
p.id_type = ty.id_type AND
p.id_agence = ag.id_agence AND
p.id_domain_is = dis.id_domain_is AND
p.id_sponsor = sp.id_sponsor AND
p.id_constructeur = cons.id_constructeur AND
p.id_groupe = gr.id_groupe AND
p.id_target_price = tarp.id_target_price AND
p.id_contrat = co.id_contrat AND
p.id_projet = his.id_projet AND
p.id_capitalisation = cap.id_capitalisation
GROUP BY p.id_projet;

Answer 1

Why the subquery? Try this:
SELECT *
FROM table1 t1
LEFT JOIN table2 t2
FORCE INDEX FOR JOIN (id_project)
ON (t2.id_project = t1.id_project);
You probably won't even need to force the index with the above query.