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doc: Add more details about pg_stat_get_xact_blocks_{fetched,hit}

The explanation describing the dependency to system read() calls forthese two functions has been removed inddfc2d9. And after morediscussion aboutd69c404, we have concluded that adding more detailsmakes them easier to understand.While on it, use the term "block read requests" (maybe found in cache)rather than "buffers fetched" and "buffer hits".Per discussion with Melanie Plageman, Kyotaro Horiguchi, BertrandDrouvot and myself.Discussion:https://postgr.es/m/CAAKRu_ZmdiScT4q83OAbfmR5AH-L5zWya3SXjaxiJvhCob-e2A@mail.gmail.comBackpatch-through: 11

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doc/src/sgml
- monitoring.sgml

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`‎doc/src/sgml/monitoring.sgml`

Lines changed: 8 additions & 4 deletions

Original file line number	Diff line number	Diff line change
`@@ -5735,8 +5735,11 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i`
`5735`	`5735`	`<returnvalue>bigint</returnvalue>`
`5736`	`5736`	`</para>`
`5737`	`5737`	`<para>`
`5738`		`- Returns the number of buffers fetched for table or index, in the current`
`5739`		`- transaction.`
	`5738`	`+ Returns the number of block read requests for table or index, in the`
	`5739`	`+ current transaction. This number minus`
	`5740`	`+ <function>pg_stat_get_xact_blocks_hit</function> gives the number of`
	`5741`	`+ kernel <function>read()</function> calls; the number of actual`
	`5742`	`+ physical reads is usually lower due to kernel-level buffering.`
`5740`	`5743`	`</para></entry>`
`5741`	`5744`	`</row>`
`5742`	`5745`
`@@ -5749,8 +5752,9 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i`
`5749`	`5752`	`<returnvalue>bigint</returnvalue>`
`5750`	`5753`	`</para>`
`5751`	`5754`	`<para>`
`5752`		`- Returns the number of buffer hits for table or index, in the current`
`5753`		`- transaction.`
	`5755`	`+ Returns the number of block read requests for table or index, in the`
	`5756`	`+ current transaction, found in cache (not triggering kernel`
	`5757`	`+ <function>read()</function> calls).`
`5754`	`5758`	`</para></entry>`
`5755`	`5759`	`</row>`
`5756`	`5760`

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`‎doc/src/sgml/monitoring.sgml`

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