{"id":923174,"date":"2024-06-09T08:10:35","date_gmt":"2024-06-09T08:10:35","guid":{"rendered":"https:\/\/proxyelite.info\/?p=923174"},"modified":"2024-06-09T08:10:36","modified_gmt":"2024-06-09T08:10:36","slug":"what-is-selectivity-in-a-database","status":"publish","type":"post","link":"https:\/\/proxyelite.info\/vi\/what-is-selectivity-in-a-database\/","title":{"rendered":"T\u00ednh ch\u1ecdn l\u1ecdc trong c\u01a1 s\u1edf d\u1eef li\u1ec7u l\u00e0 g\u00ec?"},"content":{"rendered":"

Hi\u1ec3u kh\u00e1i ni\u1ec7m v\u1ec1 t\u00ednh ch\u1ecdn l\u1ecdc trong c\u01a1 s\u1edf d\u1eef li\u1ec7u l\u00e0 r\u1ea5t quan tr\u1ecdng \u0111\u1ec3 t\u1ed1i \u01b0u h\u00f3a vi\u1ec7c truy xu\u1ea5t d\u1eef li\u1ec7u v\u00e0 \u0111\u1ea3m b\u1ea3o hi\u1ec7u su\u1ea5t c\u01a1 s\u1edf d\u1eef li\u1ec7u hi\u1ec7u qu\u1ea3. B\u00e0i vi\u1ebft n\u00e0y gi\u1ea3i th\u00edch t\u00ednh ch\u1ecdn l\u1ecdc l\u00e0 g\u00ec, t\u1ea1i sao n\u00f3 quan tr\u1ecdng c\u0169ng nh\u01b0 c\u00e1ch \u0111o l\u01b0\u1eddng v\u00e0 c\u1ea3i thi\u1ec7n t\u00ednh ch\u1ecdn l\u1ecdc.<\/p>\n\n\n\n

\u0110\u1ecbnh ngh\u0129a v\u00e0 kh\u00e1i ni\u1ec7m c\u01a1 b\u1ea3n<\/h2>\n\n\n\n

T\u00ednh ch\u1ecdn l\u1ecdc trong ng\u1eef c\u1ea3nh c\u01a1 s\u1edf d\u1eef li\u1ec7u \u0111\u1ec1 c\u1eadp \u0111\u1ebfn hi\u1ec7u qu\u1ea3 m\u00e0 truy v\u1ea5n ho\u1eb7c ch\u1ec9 m\u1ee5c c\u00f3 th\u1ec3 thu h\u1eb9p t\u00ecm ki\u1ebfm v\u00e0o m\u1ed9t t\u1eadp h\u1ee3p con d\u1eef li\u1ec7u c\u1ee5 th\u1ec3. \u0110\u00e2y l\u00e0 m\u1ed9t bi\u1ec7n ph\u00e1p quan tr\u1ecdng v\u00ec n\u00f3 \u1ea3nh h\u01b0\u1edfng tr\u1ef1c ti\u1ebfp \u0111\u1ebfn hi\u1ec7u su\u1ea5t v\u00e0 t\u1ed1c \u0111\u1ed9 ho\u1ea1t \u0111\u1ed9ng c\u1ee7a c\u01a1 s\u1edf d\u1eef li\u1ec7u.<\/p>\n\n\n\n

B\u1ea3ng: C\u00e1c kh\u00e1i ni\u1ec7m ch\u00ednh v\u1ec1 t\u00ednh ch\u1ecdn l\u1ecdc<\/h3>\n\n\n\n
\u00dd t\u01b0\u1edfng<\/th>S\u1ef1 mi\u00eau t\u1ea3<\/th><\/tr><\/thead>
T\u00ednh ch\u1ecdn l\u1ecdc cao<\/td>Cho bi\u1ebft m\u1ed9t truy v\u1ea5n ho\u1eb7c ch\u1ec9 m\u1ee5c tr\u1ea3 v\u1ec1 m\u1ed9t t\u1eadp h\u1ee3p con nh\u1ecf trong t\u1ed5ng s\u1ed1 b\u1ea3n ghi, gi\u00fap t\u00ecm ki\u1ebfm nhanh h\u01a1n.<\/td><\/tr>
\u0110\u1ed9 ch\u1ecdn l\u1ecdc th\u1ea5p<\/td>Cho bi\u1ebft m\u1ed9t truy v\u1ea5n ho\u1eb7c ch\u1ec9 m\u1ee5c tr\u1ea3 v\u1ec1 m\u1ed9t t\u1eadp h\u1ee3p con l\u1edbn trong t\u1ed5ng s\u1ed1 b\u1ea3n ghi, d\u1eabn \u0111\u1ebfn t\u00ecm ki\u1ebfm ch\u1eadm h\u01a1n.<\/td><\/tr>
Gi\u00e1 tr\u1ecb duy nh\u1ea5t<\/td>C\u00e1c c\u1ed9t c\u00f3 nhi\u1ec1u gi\u00e1 tr\u1ecb duy nh\u1ea5t c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao.<\/td><\/tr>
Gi\u00e1 tr\u1ecb l\u1eb7p l\u1ea1i<\/td>C\u00e1c c\u1ed9t c\u00f3 \u00edt gi\u00e1 tr\u1ecb duy nh\u1ea5t c\u00f3 \u0111\u1ed9 ch\u1ecdn l\u1ecdc th\u1ea5p.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

T\u1ea7m quan tr\u1ecdng c\u1ee7a t\u00ednh ch\u1ecdn l\u1ecdc<\/h2>\n\n\n\n

T\u00e1c \u0111\u1ed9ng hi\u1ec7u su\u1ea5t<\/h3>\n\n\n\n

T\u00ednh ch\u1ecdn l\u1ecdc l\u00e0 m\u1ed9t y\u1ebfu t\u1ed1 quan tr\u1ecdng trong hi\u1ec7u su\u1ea5t c\u01a1 s\u1edf d\u1eef li\u1ec7u. T\u00ednh ch\u1ecdn l\u1ecdc cao cho ph\u00e9p c\u01a1 s\u1edf d\u1eef li\u1ec7u nhanh ch\u00f3ng t\u00ecm v\u00e0 truy xu\u1ea5t c\u00e1c b\u1ea3n ghi c\u1ee5 th\u1ec3 m\u00e0 kh\u00f4ng c\u1ea7n qu\u00e9t ph\u1ea7n l\u1edbn t\u1eadp d\u1eef li\u1ec7u. \u0110i\u1ec1u n\u00e0y d\u1eabn \u0111\u1ebfn ph\u1ea3n h\u1ed3i truy v\u1ea5n nhanh h\u01a1n v\u00e0 s\u1eed d\u1ee5ng t\u00e0i nguy\u00ean hi\u1ec7u qu\u1ea3 h\u01a1n.<\/p>\n\n\n\n

Hi\u1ec7u qu\u1ea3 ch\u1ec9 s\u1ed1<\/h3>\n\n\n\n

C\u00e1c ch\u1ec9 m\u1ee5c \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 t\u0103ng t\u1ed1c c\u00e1c truy v\u1ea5n c\u01a1 s\u1edf d\u1eef li\u1ec7u. T\u00ednh ch\u1ecdn l\u1ecdc c\u1ee7a m\u1ed9t ch\u1ec9 s\u1ed1 quy\u1ebft \u0111\u1ecbnh t\u00ednh hi\u1ec7u qu\u1ea3 c\u1ee7a n\u00f3. Ch\u1ec9 m\u1ee5c c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao c\u00f3 th\u1ec3 gi\u1ea3m \u0111\u00e1ng k\u1ec3 l\u01b0\u1ee3ng d\u1eef li\u1ec7u c\u1ea7n qu\u00e9t, t\u1eeb \u0111\u00f3 t\u0103ng t\u1ed1c hi\u1ec7u su\u1ea5t truy v\u1ea5n.<\/p>\n\n\n\n

\u0110o \u0111\u1ed9 ch\u1ecdn l\u1ecdc<\/h2>\n\n\n\n

C\u00f4ng th\u1ee9c ch\u1ecdn l\u1ecdc<\/h3>\n\n\n\n

\u0110\u1ed9 ch\u1ecdn l\u1ecdc th\u01b0\u1eddng \u0111\u01b0\u1ee3c \u0111o b\u1eb1ng c\u00f4ng th\u1ee9c sau:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

T\u1ef7 l\u1ec7 n\u00e0y gi\u00fap x\u00e1c \u0111\u1ecbnh m\u1ee9c \u0111\u1ed9 thu h\u1eb9p k\u1ebft qu\u1ea3 t\u00ecm ki\u1ebfm c\u1ee7a m\u1ed9t ch\u1ec9 m\u1ee5c. T\u1ef7 l\u1ec7 cao h\u01a1n cho th\u1ea5y \u0111\u1ed9 ch\u1ecdn l\u1ecdc cao h\u01a1n v\u00e0 do \u0111\u00f3 hi\u1ec7u su\u1ea5t t\u1ed1t h\u01a1n.<\/p>\n\n\n\n

T\u00ednh to\u00e1n v\u00ed d\u1ee5<\/h3>\n\n\n\n

H\u00e3y xem x\u00e9t m\u1ed9t b\u1ea3ng c\u01a1 s\u1edf d\u1eef li\u1ec7u c\u00f3 10.000 b\u1ea3n ghi. N\u1ebfu m\u1ed9t c\u1ed9t c\u00f3 9.000 gi\u00e1 tr\u1ecb duy nh\u1ea5t th\u00ec \u0111\u1ed9 ch\u1ecdn l\u1ecdc c\u1ee7a c\u1ed9t \u0111\u00f3 l\u00e0:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

T\u00ednh ch\u1ecdn l\u1ecdc cao n\u00e0y cho th\u1ea5y c\u00e1c truy v\u1ea5n tr\u00ean c\u1ed9t n\u00e0y s\u1ebd r\u1ea5t hi\u1ec7u qu\u1ea3.<\/p>\n\n\n\n

C\u00e1c y\u1ebfu t\u1ed1 \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn t\u00ednh ch\u1ecdn l\u1ecdc<\/h2>\n\n\n\n

Ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u<\/h3>\n\n\n\n

Vi\u1ec7c ph\u00e2n ph\u1ed1i d\u1eef li\u1ec7u trong m\u1ed9t c\u1ed9t \u1ea3nh h\u01b0\u1edfng \u0111\u00e1ng k\u1ec3 \u0111\u1ebfn t\u00ednh ch\u1ecdn l\u1ecdc c\u1ee7a n\u00f3. C\u00e1c c\u1ed9t c\u00f3 gi\u00e1 tr\u1ecb duy nh\u1ea5t \u0111\u01b0\u1ee3c ph\u00e2n b\u1ed5 \u0111\u1ec1u c\u00f3 xu h\u01b0\u1edbng c\u00f3 \u0111\u1ed9 ch\u1ecdn l\u1ecdc cao h\u01a1n.<\/p>\n\n\n\n

Lo\u1ea1i ch\u1ec9 m\u1ee5c<\/h3>\n\n\n\n

C\u00e1c lo\u1ea1i ch\u1ec9 m\u1ee5c kh\u00e1c nhau (v\u00ed d\u1ee5: c\u00e2y B, ch\u1ec9 m\u1ee5c b\u0103m) c\u00f3 t\u00e1c \u0111\u1ed9ng kh\u00e1c nhau \u0111\u1ebfn t\u00ednh ch\u1ecdn l\u1ecdc. Vi\u1ec7c hi\u1ec3u lo\u1ea1i ch\u1ec9 m\u1ee5c n\u00e0o s\u1ebd s\u1eed d\u1ee5ng d\u1ef1a tr\u00ean \u0111\u1eb7c \u0111i\u1ec3m d\u1eef li\u1ec7u c\u00f3 th\u1ec3 t\u1ed1i \u01b0u h\u00f3a hi\u1ec7u su\u1ea5t.<\/p>\n\n\n\n

M\u1eabu truy v\u1ea5n<\/h3>\n\n\n\n

C\u00e1ch vi\u1ebft truy v\u1ea5n c\u0169ng c\u00f3 th\u1ec3 \u1ea3nh h\u01b0\u1edfng \u0111\u1ebfn t\u00ednh ch\u1ecdn l\u1ecdc. Vi\u1ec7c s\u1eed d\u1ee5ng ti\u00eau ch\u00ed t\u00ecm ki\u1ebfm c\u1ee5 th\u1ec3 t\u1eadn d\u1ee5ng c\u00e1c c\u1ed9t c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao c\u00f3 th\u1ec3 n\u00e2ng cao hi\u1ec7u su\u1ea5t.<\/p>\n\n\n\n

V\u00ed d\u1ee5 trong th\u1ebf gi\u1edbi th\u1ef1c<\/h2>\n\n\n\n

V\u00ed d\u1ee5 1: \u0110\u1ed9 ch\u1ecdn l\u1ecdc cao<\/h3>\n\n\n\n

C\u01a1 s\u1edf d\u1eef li\u1ec7u v\u1ec1 kh\u00e1ch h\u00e0ng bao g\u1ed3m m\u1ed9t c\u1ed9t d\u00e0nh cho \u0111\u1ecba ch\u1ec9 email. V\u00ec m\u1ed7i \u0111\u1ecba ch\u1ec9 email l\u00e0 duy nh\u1ea5t n\u00ean c\u1ed9t n\u00e0y c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao. C\u00e1c truy v\u1ea5n t\u00ecm ki\u1ebfm qua email s\u1ebd r\u1ea5t nhanh v\u00ec c\u00f3 th\u1ec3 x\u00e1c \u0111\u1ecbnh ch\u00ednh x\u00e1c b\u1ea3n ghi m\u1ed9t c\u00e1ch nhanh ch\u00f3ng.<\/p>\n\n\n\n

V\u00ed d\u1ee5 2: \u0110\u1ed9 ch\u1ecdn l\u1ecdc th\u1ea5p<\/h3>\n\n\n\n

C\u01a1 s\u1edf d\u1eef li\u1ec7u v\u1ec1 s\u1ea3n ph\u1ea9m bao g\u1ed3m m\u1ed9t c\u1ed9t d\u00e0nh cho danh m\u1ee5c (v\u00ed d\u1ee5: \u0111\u1ed3 \u0111i\u1ec7n t\u1eed, qu\u1ea7n \u00e1o). V\u00ec ch\u1ec9 c\u00f3 m\u1ed9t v\u00e0i danh m\u1ee5c v\u00e0 nhi\u1ec1u s\u1ea3n ph\u1ea9m trong m\u1ed7i danh m\u1ee5c n\u00ean c\u1ed9t n\u00e0y c\u00f3 \u0111\u1ed9 ch\u1ecdn l\u1ecdc th\u1ea5p. C\u00e1c truy v\u1ea5n t\u00ecm ki\u1ebfm theo danh m\u1ee5c s\u1ebd ch\u1eadm h\u01a1n v\u00ec ch\u00fang tr\u1ea3 v\u1ec1 t\u1eadp h\u1ee3p con d\u1eef li\u1ec7u l\u1edbn h\u01a1n.<\/p>\n\n\n\n

C\u00f4ng c\u1ee5 ph\u00e2n t\u00edch t\u00ednh ch\u1ecdn l\u1ecdc<\/h2>\n\n\n\n

H\u1ec7 th\u1ed1ng Qu\u1ea3n l\u00fd D\u1eef li\u1ec7u<\/h3>\n\n\n\n

H\u1ea7u h\u1ebft c\u00e1c H\u1ec7 th\u1ed1ng qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u (DBMS) hi\u1ec7n \u0111\u1ea1i, nh\u01b0 MySQL, PostgreSQL v\u00e0 Oracle, \u0111\u1ec1u cung c\u1ea5p c\u00e1c c\u00f4ng c\u1ee5 v\u00e0 l\u1ec7nh \u0111\u1ec3 ph\u00e2n t\u00edch v\u00e0 t\u1ed1i \u01b0u h\u00f3a t\u00ednh ch\u1ecdn l\u1ecdc. Nh\u1eefng c\u00f4ng c\u1ee5 n\u00e0y c\u00f3 th\u1ec3 gi\u00fap x\u00e1c \u0111\u1ecbnh c\u00e1c c\u1ed9t c\u00f3 \u0111\u1ed9 ch\u1ecdn l\u1ecdc th\u1ea5p v\u00e0 \u0111\u1ec1 xu\u1ea5t c\u00e1c c\u00e1ch c\u1ea3i thi\u1ec7n hi\u1ec7u su\u1ea5t.<\/p>\n\n\n\n

Tr\u00ecnh ph\u00e2n t\u00edch truy v\u1ea5n<\/h3>\n\n\n\n

M\u00e1y ph\u00e2n t\u00edch truy v\u1ea5n c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c s\u1eed d\u1ee5ng \u0111\u1ec3 ki\u1ec3m tra xem c\u00e1c truy v\u1ea5n s\u1eed d\u1ee5ng ch\u1ec9 m\u1ee5c t\u1ed1t nh\u01b0 th\u1ebf n\u00e0o. C\u00e1c c\u00f4ng c\u1ee5 nh\u01b0 EXPLAIN trong MySQL ho\u1eb7c PostgreSQL c\u00f3 th\u1ec3 hi\u1ec3n th\u1ecb k\u1ebf ho\u1ea1ch th\u1ef1c hi\u1ec7n truy v\u1ea5n v\u00e0 gi\u00fap x\u00e1c \u0111\u1ecbnh c\u00e1c t\u1eafc ngh\u1ebdn do t\u00ednh ch\u1ecdn l\u1ecdc th\u1ea5p g\u00e2y ra.<\/p>\n\n\n\n

Ph\u1ea7n k\u1ebft lu\u1eadn<\/h2>\n\n\n\n

T\u00ednh ch\u1ecdn l\u1ecdc l\u00e0 m\u1ed9t kh\u00e1i ni\u1ec7m c\u01a1 b\u1ea3n trong qu\u1ea3n l\u00fd c\u01a1 s\u1edf d\u1eef li\u1ec7u, \u1ea3nh h\u01b0\u1edfng tr\u1ef1c ti\u1ebfp \u0111\u1ebfn hi\u1ec7u su\u1ea5t truy v\u1ea5n v\u00e0 hi\u1ec7u qu\u1ea3 t\u1ed5ng th\u1ec3. Hi\u1ec3u c\u00e1ch \u0111o l\u01b0\u1eddng v\u00e0 t\u1ed1i \u01b0u h\u00f3a \u0111\u1ed9 ch\u1ecdn l\u1ecdc c\u00f3 th\u1ec3 d\u1eabn \u0111\u1ebfn nh\u1eefng c\u1ea3i ti\u1ebfn \u0111\u00e1ng k\u1ec3 trong ho\u1ea1t \u0111\u1ed9ng c\u01a1 s\u1edf d\u1eef li\u1ec7u. B\u1eb1ng c\u00e1ch t\u1eadn d\u1ee5ng c\u00e1c c\u1ed9t c\u00f3 t\u00ednh ch\u1ecdn l\u1ecdc cao v\u00e0 chi\u1ebfn l\u01b0\u1ee3c l\u1eadp ch\u1ec9 m\u1ee5c ph\u00f9 h\u1ee3p, c\u01a1 s\u1edf d\u1eef li\u1ec7u c\u00f3 th\u1ec3 \u0111\u01b0\u1ee3c \u0111i\u1ec1u ch\u1ec9nh \u0111\u1ec3 c\u00f3 hi\u1ec7u su\u1ea5t t\u1ed1t h\u01a1n, \u0111\u1ea3m b\u1ea3o truy xu\u1ea5t d\u1eef li\u1ec7u nhanh h\u01a1n v\u00e0 hi\u1ec7u qu\u1ea3 h\u01a1n.<\/p>\n\n\n\n

T\u1ed1i \u01b0u h\u00f3a t\u00ednh ch\u1ecdn l\u1ecdc kh\u00f4ng ch\u1ec9 l\u00e0 hi\u1ec3u l\u00fd thuy\u1ebft m\u00e0 c\u00f2n l\u00e0 \u00e1p d\u1ee5ng n\u00f3 b\u1eb1ng c\u00e1c c\u00f4ng c\u1ee5 v\u00e0 k\u1ef9 thu\u1eadt ph\u00f9 h\u1ee3p. Cho d\u00f9 b\u1ea1n l\u00e0 qu\u1ea3n tr\u1ecb vi\u00ean c\u01a1 s\u1edf d\u1eef li\u1ec7u hay nh\u00e0 ph\u00e1t tri\u1ec3n, vi\u1ec7c n\u1eafm v\u1eefng kh\u00e1i ni\u1ec7m v\u1ec1 t\u00ednh ch\u1ecdn l\u1ecdc s\u1ebd gi\u00fap b\u1ea1n thi\u1ebft k\u1ebf c\u01a1 s\u1edf d\u1eef li\u1ec7u hi\u1ec7u qu\u1ea3 v\u00e0 ph\u1ea3n h\u1ed3i nhanh h\u01a1n.<\/p>","protected":false},"excerpt":{"rendered":"

Understanding the concept of selectivity in databases is vital for optimizing data retrieval and ensuring efficient database performance. This article explains what selectivity is, why it is important, and how it can be measured and improved. Definition and Basics Selectivity in a database context refers to the efficiency with which a query or an index […]<\/p>\n","protected":false},"author":1,"featured_media":923177,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[1],"tags":[],"class_list":["post-923174","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-articles"],"acf":[],"_links":{"self":[{"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/posts\/923174","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/comments?post=923174"}],"version-history":[{"count":0,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/posts\/923174\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/media\/923177"}],"wp:attachment":[{"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/media?parent=923174"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/categories?post=923174"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/proxyelite.info\/vi\/wp-json\/wp\/v2\/tags?post=923174"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}