{"id":75678,"date":"2020-03-27T19:42:27","date_gmt":"2020-03-27T17:42:27","guid":{"rendered":"https:\/\/prohoster.info\/blog\/administrirovanie\/o-chem-molchit-explain-i-kak-ego-razgovorit"},"modified":"2020-03-27T19:42:27","modified_gmt":"2020-03-27T17:42:27","slug":"o-chem-molchit-explain-i-kak-ego-razgovorit","status":"publish","type":"post","link":"https:\/\/prohoster.info\/cs\/blog\/administrirovanie\/o-chem-molchit-explain-i-kak-ego-razgovorit","title":{"rendered":"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p>Klasick\u00fd dotaz, kter\u00fd v\u00fdvoj\u00e1\u0159 t\u00e9m\u011b\u0159 v\u017edy pokl\u00e1d\u00e1 sv\u00e9mu DBA nebo majiteli firmy \u2013 konzultantovi pro PostgreSQL, zn\u00ed: <i>\u201ePro\u010d se dotazy na datab\u00e1zi vykon\u00e1vaj\u00ed tak dlouho?\u201c<\/i><\/p>\n<p>Tradi\u010dn\u00ed soubor p\u0159\u00ed\u010din:<\/p>\n<ul>\n<li><b>neefektivn\u00ed algoritmus<\/b><br \/>\nkdy\u017e se rozhodnete prov\u00e9st JOIN n\u011bkolika CTE s des\u00edtkami tis\u00edc z\u00e1znam\u016f<\/li>\n<li><b>Tento moment je velmi aktu\u00e1ln\u00ed pr\u00e1v\u011b pro PostgreSQL, kdy\u017e jste velk\u00fd dataset 'nalili' na server, d\u011bl\u00e1te dotaz \u2013 a on v\u00e1m 'sek\u00e1' po tabulce. Proto\u017ee v\u010dera tam bylo 10 z\u00e1znam\u016f, a dnes 10 milion\u016f, ale PostgreSQL o tom je\u0161t\u011b nev\u00ed, a je t\u0159eba mu to nazna\u010dit. [<\/b><br \/>\npokud se skute\u010dn\u00e9 rozlo\u017een\u00ed dat v tabulce ji\u017e v\u00fdrazn\u011b li\u0161\u00ed od toho, kter\u00e9 bylo shrom\u00e1\u017ed\u011bno ANALYZE naposledy<\/li>\n<li><b>Um\u00edstili jste velkou a zat\u00ed\u017eenou datab\u00e1zi na slab\u00fd server, kter\u00fd postr\u00e1d\u00e1 disk, pam\u011b\u0165 a v\u00fdkonnost samotn\u00e9ho procesoru. A to je v\u0161e\u2026 N\u011bkde je strop v\u00fdkonnosti, nad kter\u00fd u\u017e nem\u016f\u017eete sko\u010dit.<\/b><br \/>\na ji\u017e nesta\u010d\u00ed vyhrazen\u00e9 v\u00fdpo\u010detn\u00ed prost\u0159edky CPU, neust\u00e1le se zvy\u0161uj\u00ed gigabajty pam\u011bti nebo disk nesta\u010d\u00ed na v\u0161echny \u201epo\u017eadavky\u201c datab\u00e1ze<\/li>\n<li><b>blokace<\/b> od konkuren\u010dn\u00edch proces\u016f<\/li>\n<\/ul>\n<p>\nA pokud jsou z\u00e1mky dost slo\u017eit\u00e9 na zachycen\u00ed a anal\u00fdzu, pro v\u0161echno ostatn\u00ed n\u00e1m sta\u010d\u00ed <b>pl\u00e1n dotazu<\/b>, kter\u00fd lze z\u00edskat pomoc\u00ed <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/using-explain\">p\u0159\u00edkazu EXPLAIN<\/a><\/noindex> (<i>je lep\u0161\u00ed rovnou pou\u017e\u00edt EXPLAIN (ANALYZE, BUFFERS) \u2026<\/i>) nebo <noindex><a rel=\"nofollow\" href=\"https:\/\/postgrespro.ru\/docs\/postgresql\/12\/auto-explain\">modulu auto_explain<\/a><\/noindex>.<\/p>\n<p>Ale jak je uvedeno v t\u00e9\u017ee dokumentaci, <\/p>\n<blockquote><p>\u201ePochopen\u00ed pl\u00e1nu je um\u011bn\u00ed, a abyste se ho nau\u010dili, je pot\u0159eba ur\u010dit\u00e9 zku\u0161enosti, \u2026\u201c<\/p><\/blockquote>\n<p> Ale m\u016f\u017eete se obej\u00edt i bez n\u011bj, pokud pou\u017eijete vhodn\u00fd n\u00e1stroj!<br \/>\n<noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><br \/>\nJak obvykle vypad\u00e1 pl\u00e1n dotazu? N\u011bjak takto:<\/p>\n<pre><code class=\"plaintext\">Index Scan using pg_class_relname_nsp_index on pg_class (skute\u010dn\u00fd \u010das=0.049..0.050 \u0159\u00e1dky=1 smy\u010dky=1)\n  Index Podm\u00ednka: (relname = $1)\n  Filtr: (oid = $0)\n  Buffers: sd\u00edlen\u00fd hit=4\n  InitPlan 1 (vrac\u00ed $0,$1)\n    -&gt; Limit (skute\u010dn\u00fd \u010das=0.019..0.020 \u0159\u00e1dky=1 smy\u010dky=1)\n          Buffers: sd\u00edlen\u00fd hit=1\n          -&gt; Seq Scan on pg_class pg_class_1 (skute\u010dn\u00fd \u010das=0.015..0.015 \u0159\u00e1dky=1 smy\u010dky=1)\n                Filtr: (relkind = 'r'::\"char\")\n                \u0158\u00e1dky odstran\u011bn\u00e9 filtrem: 5\n                Buffers: sd\u00edlen\u00fd hit=1<\/code><\/pre>\n<p>\nnebo takto:<\/p>\n<pre><code class=\"plaintext\">\"Append  (n\u00e1klady=868.60..878.95 \u0159\u00e1dky=2 \u0161\u00ed\u0159ka=233) (skute\u010dn\u00fd \u010das=0.024..0.144 \u0159\u00e1dky=2 smy\u010dky=1)\"\n\"  Buffers: sd\u00edlen\u00fd hit=3\"\n\"  CTE cl\"\n\"    -&gt;  Seq Scan on pg_class  (n\u00e1klady=0.00..868.60 \u0159\u00e1dky=9972 \u0161\u00ed\u0159ka=537) (skute\u010dn\u00fd \u010das=0.016..0.042 \u0159\u00e1dky=101 smy\u010dky=1)\"\n\"          Buffers: sd\u00edlen\u00fd hit=3\"\n\"  -&gt;  Limit  (n\u00e1klady=0.00..0.10 \u0159\u00e1dky=1 \u0161\u00ed\u0159ka=233) (skute\u010dn\u00fd \u010das=0.023..0.024 \u0159\u00e1dky=1 smy\u010dky=1)\"\n\"        Buffers: sd\u00edlen\u00fd hit=1\"\n\"        -&gt;  CTE Scan on cl  (n\u00e1klady=0.00..997.20 \u0159\u00e1dky=9972 \u0161\u00ed\u0159ka=233) (skute\u010dn\u00fd \u010das=0.021..0.021 \u0159\u00e1dky=1 smy\u010dky=1)\"\n\"              Buffers: sd\u00edlen\u00fd hit=1\"\n\"  -&gt;  Limit  (n\u00e1klady=10.00..10.10 \u0159\u00e1dky=1 \u0161\u00ed\u0159ka=233) (skute\u010dn\u00fd \u010das=0.117..0.118 \u0159\u00e1dky=1 smy\u010dky=1)\"\n\"        Buffers: sd\u00edlen\u00fd hit=2\"\n\"        -&gt;  CTE Scan on cl cl_1  (n\u00e1klady=0.00..997.20 \u0159\u00e1dky=9972 \u0161\u00ed\u0159ka=233) (skute\u010dn\u00fd \u010das=0.001..0.104 \u0159\u00e1dky=101 smy\u010dky=1)\"\n\"              Buffers: sd\u00edlen\u00fd hit=2\"\n\"Pl\u00e1novac\u00ed \u010das: 0.634 ms\"\n\"\u010cas prov\u00e1d\u011bn\u00ed: 0.248 ms\"<\/code><\/pre>\n<p>\nAle \u010d\u00edst pl\u00e1n textem \u201ez pap\u00edru\u201c je velmi slo\u017eit\u00e9 a nep\u0159ehledn\u00e9:<\/p>\n<ul>\n<li>v uzlu se zobrazuje <b>sou\u010det zdroj\u016f podstromu<\/b><br \/>\nto znamen\u00e1, abyste pochopili, kolik \u010dasu bylo pot\u0159ebn\u00e9 k vykon\u00e1n\u00ed konkr\u00e9tn\u00edho uzlu, nebo kolik p\u0159esn\u011b to \u010dten\u00ed z tabulky zvedlo dat z disku \u2013 mus\u00edte n\u011bjak ode\u010d\u00edst jedno od druh\u00e9ho<\/li>\n<li>\u010das uzlu je pot\u0159eba <b>n\u00e1sobit cykly<\/b><br \/>\nAno, od\u010d\u00edt\u00e1n\u00ed nen\u00ed v\u016fbec ta nejslo\u017eit\u011bj\u0161\u00ed operace, kterou je t\u0159eba prov\u00e1d\u011bt \u201ev hlav\u011b\u201c \u2014 v\u017edy\u0165 \u010das proveden\u00ed je uveden pr\u016fm\u011brn\u011b pro jedno proveden\u00ed uzlu, a t\u011bch m\u016f\u017ee b\u00fdt stovky.<\/li>\n<li>No a to v\u0161e dohromady br\u00e1n\u00ed odpov\u011bd\u011bt na z\u00e1kladn\u00ed ot\u00e1zku \u2014 kdo vlastn\u011b. <b>\u201eSlab\u00fd \u010dl\u00e1nek.\u201c<\/b>?<\/li>\n<\/ul>\n<p>\nKdy\u017e jsme se pokusili vysv\u011btlit to n\u011bkolika stovk\u00e1m na\u0161ich v\u00fdvoj\u00e1\u0159\u016f, pochopili jsme, \u017ee zven\u010d\u00ed to vypad\u00e1 p\u0159ibli\u017en\u011b takto:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/abb07e902639b5ec245851aa4a0edb58.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<br \/>\nA tedy, pot\u0159ebujeme...<\/p>\n<h2>N\u00e1klad<\/h2>\n<p>\nSna\u017eili jsme se shrom\u00e1\u017edit v\u0161echny kl\u00ed\u010dov\u00e9 mechanismy, kter\u00e9 pom\u00e1haj\u00ed podle pl\u00e1nu a dotazu pochopit, \u201ekdo je vin\u00edk a co d\u011blat\u201c. A tak\u00e9 sd\u00edlet \u010d\u00e1st sv\u00e9ho zku\u0161enosti se komunitou.<br \/>\nP\u0159edstavujeme a vyu\u017e\u00edvejte \u2014 <noindex><a rel=\"nofollow\" href=\"https:\/\/explain.tensor.ru\/\">explain.tensor.ru<\/a><\/noindex> <\/p>\n<h3>P\u0159ehlednost pl\u00e1n\u016f<\/h3>\n<p>\nJe snadn\u00e9 pochopit pl\u00e1n, kdy\u017e vypad\u00e1 takto?<\/p>\n<pre><code class=\"plaintext\">Seq Scan on pg_class (skute\u010dn\u00fd \u010das=0.009..1.304 \u0159\u00e1dk\u016f=6609 smy\u010dek=1)\n  Buffers: shared hit=263\nPlanning Time: 0.108 ms\nExecution Time: 1.800 ms\n<\/code><\/pre>\n<p>\nNeboli.<\/p>\n<p>Ale takto, <b>ve zkr\u00e1cen\u00e9 podob\u011b<\/b>, kdy\u017e jsou kl\u00ed\u010dov\u00e9 ukazatele odd\u011bleny \u2014 u\u017e to je mnohem srozumiteln\u011bj\u0161\u00ed:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/18404ce800de847f1934ac2c7fef10be.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<br \/>\nAle pokud je pl\u00e1n slo\u017eit\u011bj\u0161\u00ed \u2014 pom\u016f\u017ee n\u00e1m <b>kol\u00e1\u010dov\u00fd graf rozlo\u017een\u00ed \u010dasu<\/b> podle uzl\u016f:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/ca9fdd07278801e4f3ad15e997efc1a0.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<br \/>\nA pro ty nejobt\u00ed\u017en\u011bj\u0161\u00ed varianty p\u0159ich\u00e1z\u00ed na pomoc <b>graf v\u00fdkonu<\/b>:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/c03f358858206d7ccae20fd8142baa8c.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<br \/>\nNap\u0159\u00edklad existuj\u00ed dost netrivi\u00e1ln\u00ed situace, kdy pl\u00e1n m\u016f\u017ee m\u00edt v\u00edce ne\u017e jeden skute\u010dn\u00fd ko\u0159en:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/ea4f3f498737e9cc3f8f40c16a5bd1d7.png\" style=\"display:block;margin: 0 auto;\" \/><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/267273e8491d6b4403442672612c33ff.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<\/p>\n<h3>Struktur\u00e1ln\u00ed n\u00e1pov\u011bdy<\/h3>\n<p>\nPokud je cel\u00e1 struktura pl\u00e1nu a jeho slab\u00e1 m\u00edsta ji\u017e rozlo\u017eena a viditeln\u00e1 \u2014 pro\u010d je neozna\u010dit v\u00fdvoj\u00e1\u0159i a nevysv\u011btlit \u201ed\u011btsk\u00fdm jazykem\u201c?<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/0e7157a09f7f5707e769cc1d6a8f3af8.png\" style=\"display:block;margin: 0 auto;\" \/>Takov\u00fdch vzor\u016f doporu\u010den\u00ed jsme ji\u017e shrom\u00e1\u017edili n\u011bkolik des\u00edtek.<\/p>\n<h3>Postrov\u00fd profiler dotazu<\/h3>\n<p>\nNyn\u00ed, pokud na analyzovan\u00fd pl\u00e1n aplikujete p\u016fvodn\u00ed dotaz, uvid\u00edte, kolik \u010dasu bylo v\u011bnov\u00e1no ka\u017ed\u00e9mu jednotliv\u00e9mu oper\u00e1toru \u2014 p\u0159ibli\u017en\u011b takto:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/0766d331798d529363d10006ace211f3.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<br \/>\n\u2026 nebo dokonce takto:<\/p>\n<p><img decoding=\"async\" alt=\"O \u010dem ml\u010d\u00ed EXPLAIN a jak ho p\u0159im\u011bt k rozhovoru\" src=\"\/wp-content\/uploads\/2020\/03\/4efd23d84792c5a7f54b40e86ff17353.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\n<\/p>\n<h3>Dosazen\u00ed parametr\u016f do dotazu<\/h3>\n<p>\nPokud jste \u201ep\u0159ipojili\u201c k pl\u00e1nu nejen dotaz, ale i jeho parametry z DETAIL_\u0159\u00e1dku logu, m\u016f\u017eete jej nav\u00edc zkop\u00edrovat v jedn\u00e9 z variant:<\/p>\n<ul>\n<li>s dosazen\u00edm hodnot do dotazu<br \/>\npro p\u0159\u00edm\u00e9 proveden\u00ed na sv\u00e9 datab\u00e1zi a n\u00e1slednou profilaci<\/p>\n<pre><code class=\"sql\">SELECT 'const', 'param'::text;<\/code><\/pre>\n<\/li>\n<li>s dosazen\u00edm hodnot p\u0159es PREPARE\/EXECUTE<br \/>\npro simulaci pr\u00e1ce pl\u00e1nova\u010de, kdy m\u016f\u017ee b\u00fdt parametrick\u00e1 \u010d\u00e1st ignorov\u00e1na \u2014 nap\u0159\u00edklad p\u0159i pr\u00e1ci s partitionovan\u00fdmi tabulkami<\/p>\n<pre><code class=\"sql\">DEALLOCATE ALL;\nPREPARE q(text) AS SELECT 'const', $1::text;\nEXECUTE q('param'::text);\n<\/code><\/pre>\n<\/li>\n<\/ul>\n<p><\/p>\n<h3>Archiv pl\u00e1n\u016f<\/h3>\n<p>\nVkl\u00e1dejte, analyzujte, sd\u00edlejte se sv\u00fdmi kolegy! Pl\u00e1ny z\u016fstanou v archivu a budete se k nim moci vr\u00e1tit pozd\u011bji: <noindex><a rel=\"nofollow\" href=\"https:\/\/explain.tensor.ru\/archive\/\">explain.tensor.ru\/archive<\/a><\/noindex><\/p>\n<p>Ale pokud nechcete, aby v\u00e1\u0161 pl\u00e1n vid\u011bli ostatn\u00ed, nezapome\u0148te za\u0161krtnout pol\u00ed\u010dko \u201enepublikovat v archivu\u201c.<\/p>\n<p>V n\u00e1sleduj\u00edc\u00edch \u010dl\u00e1nc\u00edch se pod\u011bl\u00edm o ty slo\u017eitosti a \u0159e\u0161en\u00ed, kter\u00e9 se p\u0159i anal\u00fdze pl\u00e1nu objevuj\u00ed.<br \/>\n<br \/>Zdroj: <a content=\"nofollow\" rel=\"nofollow\" href=\"https:\/\/habr.com\/ru\/company\/tensor\/blog\/477624\/\">habr.com<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"<p>\u041a\u043b\u0430\u0441\u0441\u0438\u0447\u0435\u0441\u043a\u0438\u0439 \u0432\u043e\u043f\u0440\u043e\u0441, \u0441 \u043a\u043e\u0442\u043e\u0440\u044b\u043c \u0440\u0430\u0437\u0440\u0430\u0431\u043e\u0442\u0447\u0438\u043a \u043f\u0440\u0438\u0445\u043e\u0434\u0438\u0442 \u043a \u0441\u0432\u043e\u0435\u043c\u0443 DBA \u0438\u043b\u0438 \u0432\u043b\u0430\u0434\u0435\u043b\u0435\u0446 \u0431\u0438\u0437\u043d\u0435\u0441\u0430 \u2014 \u043a \u043a\u043e\u043d\u0441\u0443\u043b\u044c\u0442\u0430\u043d\u0442\u0443 \u043f\u043e PostgreSQL, \u043f\u043e\u0447\u0442\u0438 \u0432\u0441\u0435\u0433\u0434\u0430 \u0437\u0432\u0443\u0447\u0438\u0442 \u043e\u0434\u0438\u043d\u0430\u043a\u043e\u0432\u043e: \u00ab\u041f\u043e\u0447\u0435\u043c\u0443 \u0437\u0430\u043f\u0440\u043e\u0441\u044b \u0432\u044b\u043f\u043e\u043b\u043d\u044f\u044e\u0442\u0441\u044f \u043d\u0430 \u0431\u0430\u0437\u0435 \u0442\u0430\u043a \u0434\u043e\u043b\u0433\u043e?\u00bb \u0422\u0440\u0430\u0434\u0438\u0446\u0438\u043e\u043d\u043d\u044b\u0439 \u043d\u0430\u0431\u043e\u0440 \u043f\u0440\u0438\u0447\u0438\u043d: \u043d\u0435\u044d\u0444\u0444\u0435\u043a\u0442\u0438\u0432\u043d\u044b\u0439 \u0430\u043b\u0433\u043e\u0440\u0438\u0442\u043c \u043a\u043e\u0433\u0434\u0430 \u0432\u044b \u0440\u0435\u0448\u0438\u043b\u0438 \u0441\u0434\u0435\u043b\u0430\u0442\u044c JOIN \u043d\u0435\u0441\u043a\u043e\u043b\u044c\u043a\u0438\u0445 CTE \u043f\u043e \u043f\u0430\u0440\u0435 \u0434\u0435\u0441\u044f\u0442\u043a\u043e\u0432 \u0442\u044b\u0441\u044f\u0447 \u0437\u0430\u043f\u0438\u0441\u0435\u0439 \u043d\u0435\u0430\u043a\u0442\u0443\u0430\u043b\u044c\u043d\u0430\u044f \u0441\u0442\u0430\u0442\u0438\u0441\u0442\u0438\u043a\u0430 \u0435\u0441\u043b\u0438 \u0444\u0430\u043a\u0442\u0438\u0447\u0435\u0441\u043a\u043e\u0435 \u0440\u0430\u0441\u043f\u0440\u0435\u0434\u0435\u043b\u0435\u043d\u0438\u0435 \u0434\u0430\u043d\u043d\u044b\u0445 \u0432 \u0442\u0430\u0431\u043b\u0438\u0446\u0435 \u0443\u0436\u0435 \u0441\u0438\u043b\u044c\u043d\u043e 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