{"id":4097,"date":"2013-02-21T17:41:46","date_gmt":"2013-02-21T16:41:46","guid":{"rendered":"http:\/\/blog.capdata.fr\/?p=4097"},"modified":"2019-09-13T14:14:02","modified_gmt":"2019-09-13T13:14:02","slug":"jointure-entre-syssegments-et-sysusages","status":"publish","type":"post","link":"https:\/\/blog.capdata.fr\/index.php\/jointure-entre-syssegments-et-sysusages\/","title":{"rendered":"Jointure entre syssegments et sysusages"},"content":{"rendered":"\"twitter\"<\/a>\"linkedin\"<\/a>\"mail\"<\/a>

Un segment peut s’\u00e9taler sur plusieurs fragments.\u00a0Les segments sont list\u00e9s dans syssegments.<\/p>\n

Un fragment peut supporter plusieurs segments.\u00a0Les fragments sont list\u00e9s dans master..sysusages.<\/p>\n

La jointure entre ces deux tables aurait donc du faire intervenir une troisi\u00e8me table assurant la liaison.<\/p>\n

Au lieu de cela, Sybase opte pour une jointure par bitmask<\/strong>.<\/p>\n

Dans la table\u00a0syssegments, chaque segment poss\u00e8de un num\u00e9ro.<\/p>\n

Dans la table\u00a0master..sysusages, chaque fragment poss\u00e8de une valeur num\u00e9rique qui s’appelle segmap<\/strong>. Ce nombre est la somme des puissances de deux correspondant aux segments attach\u00e9s \u00e0 ce fragment.<\/p>\n

Exemple A : Si un fragment est attach\u00e9 aux segments 0 et 1, le segmap = 2^0<\/strong>+ 2^1<\/strong>\u00a0= 3.<\/p>\n

Exemple B : Si un fragment est attach\u00e9 aux segments 2 et 5, le segmap = 2^2<\/strong>\u00a0+ 2^5<\/strong>\u00a0= 36.<\/p>\n

Se pose donc le probl\u00e8me de la lisibilit\u00e9 d’une telle m\u00e9thode. La jointure n\u00e9cessaire pour d\u00e9coder cela est la suivante :<\/p>\n

select * from syssegments, master..sysusages
\nwhere segmap & power(2,segment) = power(2,segment)<\/code><\/p>\n

On peut r\u00e9tr\u00e9cir son r\u00e9sultat pour ne voir que ce qui concerne la base courante :<\/p>\n

select * from syssegments, master..sysusages
\nwhere segmap & power(2,segment) = power(2,segment)
\nand dbid=db_id()<\/code><\/p>\n

Enfin, on peut rajouter les noms des devices pour \u00eatre plus complet :<\/p>\n

select s.name segment, d.name device, phyname
\nfrom syssegments s, master..sysusages f, master..sysdevices d
\nwhere segmap & power(2,segment) = power(2,segment)
\nand dbid=db_id()
\nand f.vdevno = d.vdevno
\norder by s.name<\/code><\/p>\n\"twitter\"<\/a>\"linkedin\"<\/a>\"mail\"<\/a>","protected":false},"excerpt":{"rendered":"

Un segment peut s’\u00e9taler sur plusieurs fragments.\u00a0Les segments sont list\u00e9s dans syssegments. Un fragment peut supporter plusieurs segments.\u00a0Les fragments sont list\u00e9s dans master..sysusages. La jointure entre ces deux tables aurait donc du faire intervenir une troisi\u00e8me table assurant la liaison.… Continuer la lecture →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":7874,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[6],"tags":[],"class_list":["post-4097","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-sybase"],"yoast_head":"\nJointure entre syssegments et sysusages - Capdata TECH BLOG<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.capdata.fr\/index.php\/jointure-entre-syssegments-et-sysusages\/\" \/>\n<meta property=\"og:locale\" content=\"fr_FR\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Jointure entre syssegments et sysusages - Capdata TECH BLOG\" \/>\n<meta property=\"og:description\" content=\"Un segment peut s’\u00e9taler sur plusieurs fragments.\u00a0Les segments sont list\u00e9s dans syssegments. 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