mysql explain使用方法

<table class='msonormaltable' cellspacing='0' border='0' style='width: 441.9pt; border-collapse: collapse;' cellpadding='0' width='589'><tbody><tr style='height: 19.5pt;'><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 33.3pt; padding-top: 0cm; height: 19.5pt; background-color: transparent; border-color: #ece9d8;' width='44'><p class='msonormal' align='left' style='margin: 0cm 0cm 0pt; text-align: left;'><span lang='en-us' style=''>table</span></td><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 35.1pt; padding-top: 0cm; height: 19.5pt; background-color: transparent; border-color: #ece9d8;' width='47'>
<table class='msonormaltable' cellspacing='0' border='0' style='margin: auto auto auto 5.4pt; width: 441pt; border-collapse: collapse;' cellpadding='0' width='588'><tbody><tr style='height: 14.25pt;'><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 81pt; padding-top: 0cm; height: 14.25pt; background-color: transparent; border-color: #ece9d8;' width='108'><p class='msonormal' align='left' style='margin: 0cm 0cm 0pt; text-align: left;'><span lang='en-us' style=''>table</span></td><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 360pt; padding-top: 0cm; height: 14.25pt; background-color: transparent; border-color: #ece9d8;' width='480'>
<table class='msonormaltable' cellspacing='0' border='0' style='border-collapse: collapse;' cellpadding='0'><tbody><tr style='height: 14.25pt;'><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 28.3%; padding-top: 0cm; height: 14.25pt; background-color: transparent; border-color: #ece9d8;' width='28%'><p class='msonormal' align='left' style='margin: 0cm 0cm 0pt; text-align: left;'><span lang='en-us' style=''>distinct</span></td><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 71.7%; padding-top: 0cm; height: 14.25pt; background-color: transparent; border-color: #ece9d8;' width='71%'>
<table class='msonormaltable' cellspacing='0' border='0' style='margin: auto auto auto 5.4pt; width: 441pt; border-collapse: collapse;' cellpadding='0' width='588'><tbody><tr style='height: 28.5pt;'><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 46.8pt; padding-top: 0cm; height: 28.5pt; background-color: transparent; border-color: #ece9d8;' width='62'><p class='msonormal' align='left' style='margin: 0cm 0cm 0pt; text-align: left;'><span lang='en-us' style=''>system</span></td><td valign='bottom' style='padding-right: 5.4pt; padding-left: 5.4pt; padding-bottom: 0cm; width: 394.2pt; padding-top: 0cm; height: 28.5pt; background-color: transparent; border-color: #ece9d8;' width='526'></td></tr></tbody></table><p/>