45 lines
3.1 KiB
Plaintext
45 lines
3.1 KiB
Plaintext
// Select random node :
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// Comparaison de plusieurs méthodes :
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// Note : calculer un count(*) prend très longtemps, il vaut mieux faire un max(eid).
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// rapide, mais ne renvoie rien si on tombe sur un trou dans les eid, probabilité 105/229894 ≈ 1/2000
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select * from node where eid = (abs(random()) % (select max(eid) from node))+1;
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// assez rapide, renvoie le noeud 1 si on tombe sur un trou dans les eid
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select * from node where eid = (abs(random()) % (select max(eid) from node))+1 or eid = (select min(eid) from node where eid > 0) order by eid desc limit 1;
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// peu rapide, trouve toujours un row, aléatoire
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select * from node limit 1 offset (abs(random()) % (select count(*)+1 from node));
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// Comparaison des temps d'exécution des méthodes ci-dessus
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// Note : les deux premiers sont sur 10000 exécutions, le dernier sur 100
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> time yes "select * from node where eid = (abs(random()) % (select max(eid) from node));" | head -n 10000 | sqlite3 db > /dev/null
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yes 0,01s user 0,00s system 0% cpu 3,272 total
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head -n 10000 0,00s user 0,02s system 0% cpu 3,283 total
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sqlite3 db > /dev/null 2,70s user 0,52s system 89% cpu 3,586 total
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> time yes "select * from node where eid = (abs(random()) % (select max(eid) from node)) or eid = (select min(eid) from node where eid > 0) order by eid desc limit 1;" | head -n 10000 | sqlite3 db > /dev/null
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yes 0,01s user 0,01s system 0% cpu 7,324 total
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head -n 10000 0,01s user 0,02s system 0% cpu 7,340 total
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sqlite3 db > /dev/null 5,94s user 0,62s system 85% cpu 7,645 total
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> time yes "select * from node limit 1 offset (abs(random()) % (select count(*) from node));" | head -n 100 | sqlite3 db > /dev/null
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yes 0,01s user 0,00s system 33% cpu 0,024 total
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head -n 100 0,00s user 0,00s system 37% cpu 0,021 total
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sqlite3 db > /dev/null 7,40s user 5,99s system 89% cpu 14,984 total
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// Avec un index sur node(eid), nettement meilleur pour les deux premières méthodes
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// Les deux premières méthodes ne bougent pas, la troisième est pas mal améliorée.
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create index i_node_eid on node(eid);
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> time yes "select * from node where eid = (abs(random()) % (select max(eid) from node));" | head -n 10000 | sqlite3 db > /dev/null
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yes 0,01s user 0,01s system 0% cpu 3,421 total
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head -n 10000 0,00s user 0,02s system 0% cpu 3,439 total
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sqlite3 db > /dev/null 2,82s user 0,53s system 89% cpu 3,752 total
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> time yes "select * from node where eid = (abs(random()) % (select max(eid) from node)) or eid = (select min(eid) from node where eid > 0) order by eid desc limit 1;" | head -n 10000 | sqlite3 db > /dev/null
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yes 0,02s user 0,01s system 0% cpu 7,059 total
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head -n 10000 0,02s user 0,02s system 0% cpu 7,076 total
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sqlite3 db > /dev/null 6,10s user 0,54s system 89% cpu 7,390 total
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> time yes "select * from node limit 1 offset (abs(random()) % (select count(*) from node));" | head -n 100 | sqlite3 db > /dev/null
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yes 0,00s user 0,01s system 25% cpu 0,031 total
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head -n 100 0,00s user 0,00s system 27% cpu 0,029 total
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sqlite3 db > /dev/null 5,71s user 3,78s system 86% cpu 10,917 total
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