## optimization – PostgreSQL does not use the index on queries returning a large number of records

I have a table called `my_table` which contains about 30 million discs. The table has an index defined on `column_a`. However, I notice that when the number of returned queries is high, the index is not used.

Index not used:

``````explain (buffers, analyze) select * from my_table where column_a in (1);
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------
Seq Scan on my_table  (cost=0.00..1306095.85 rows=7772384 width=648) (actual time=0.009..6231.312 rows=7720995 loops=1)
Filter: (column_a = 1)
Rows Removed by Filter: 7398657
Planning time: 0.840 ms
Execution time: 7717.923 ms
(6 rows)
``````

Index used:

``````explain (buffers, analyze) select * from my_table where column_a in (8);
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on my_table  (cost=1378.58..233236.51 rows=73567 width=648) (actual time=14.258..67.795 rows=74756 loops=1)
Recheck Cond: (column_a = 8)
Heap Blocks: exact=36425
Buffers: shared hit=36632
->  Bitmap Index Scan on my_table_column_a_idx  (cost=0.00..1360.19 rows=73567 width=0) (actual time=8.595..8.596 rows=74756 loops=1)
Index Cond: (column_a = 8)
Buffers: shared hit=207
Planning time: 0.855 ms
Execution time: 82.253 ms
(9 rows)
``````

I've sucked the table:

``````select relallvisible, relpages, relallvisible/relpages as ratio from pg_class where relname='my_table';
relallvisible | relpages | ratio
---------------+----------+-------
1117140 |  1117140 |     1
(1 row)
``````

## web design – How to measure phone number views without annoying the user?

Here is an appropriate native JavaScript solution:

Create a link to a clickable phone number:

``````094 834 73 843
``````

Add a function to hide the last 4 characters and display them by clicking. In addition, there is a timeout event to explain to the user what to do next.

``````function reveal_number_init() {
let phoneNumbers = document.getElementsByClassName('reveal-number');

let replaceLastNChars = function(str, replace, num) {
return str.slice(0, -num) + Array(num + 1).join(replace);
};

for (i = 0; i < phoneNumbers.length; i++) {
var element = phoneNumbers(i);

element.href = 'javascript:void(0);';
element.innerText = replaceLastNChars(element.innerText, '*', 4);

element.innerText = element.dataset.number;

setTimeout(function () {
element.href = 'tel:' + element.dataset.number;
element.innerHTML += 'Click again to call.';
}, 1000);
});
}
}

// Initialise the function:
if (document.querySelector('.reveal-number')) {
reveal_number_init();
}
});
``````

Then you can use an AJAX call to record clicks on phone numbers, depending on your content management system or database structure.

Find out how it works and how it works here – How to track and measure phone number displays with JavaScript

## Number Theory – How does the Chinese Remainder Theorem help prove \$ phi (ab) = phi (a) phi (b) \$?

I understand that the one-to-one correspondence refers to an element of set A, for example by mapping a single element distinct from set B. How does this one-to-one correspondence appear after applying the remainder theorem? Chinese to the system of equations of the referred image? ?

## Number Theory – Are these two sets of integers defined by the combination disjoint?

Fix an integer $$n geq 8$$. For each integer $$i leq n / 2$$, note by $$X_i$$ l & # 39; together
$$X_i = left { frac {n-i + 1-k} {n-i + 1} binom nk binom {k-1} {i-1} ~ middle | ~~ i leq k leq ni right }.$$

The question:

Are the sets $$X_1$$ and $$X_2$$ disjoint?

The reason:

The product $$frac {n-i + 1-k} {n-i + 1} binom nk binom {k-1} {i-1}$$ is the degree in the symmetric group $$S_n$$ the irreducible character that corresponds to the partition $$(n-k, i, 1 ^ {k-i})$$ (this last entry means the cell $$1$$ say again $$k-i$$ times), and a guess I'm working on is moving forward substantially if I get the full range of values ​​from $$X_1 cup X_2$$ without having to worry about whether or not I have repetition by pulling something out $$X_1$$ then something $$X_2$$.

The bonus question:

What I really want to be true, is that $$X_1$$, $$X_2$$, and $$X_3$$ are disjoint in pairs, so proof that covers the 3 pairs simultaneously would be the golden rule, but I would not be surprised at all if $$X_i, X_j$$ pair had something special that required individualized effort. For example, a reason $$n geq 8$$ is necessary is that sets $$X_2$$ and $$X_3$$ have corresponding partitions $$(5.2)$$ and $$(4.3)$$ of $$n = 7$$ both of which give the integer $$14$$; they are not disjointed.

Potentially relevant information:

I gladly note that in this indexing, both $$X_1$$ and $$X_2$$ have an internal redundancy, that is, if we consider them as multisets, there would be several appearances of some integers. I only talk about enter sets, not in a given set.

My work:

The computer goes up in the $$n = 3000$$ range say that they are disjointed at least as far.

## computer architecture – Why does the left arithmetic shift of the negative number lead to a positive number?

According to this Wikipedia article, when the arithmetic left shift operation is applied to a signed number, the number is multiplied by 2. However, there are certain situations where a negative number becomes a positive number when a Arithmetic left shift is applied.

Ex: Take a complement to 2, a signed integer -5 and 5 bits are used to represent it.

``````11011 ==> -5
10110 ==> -10 (-5x2)
01100 ==> +24 (?)
``````

So, after two arithmetic left turns, -5 became 24. I was waiting at -20. why is this the case?

## Stock inventory Save after the number of the observer

This watcher only fires when Quantity is out of stock or when we record Quantity. Is there another watcher in magento when the quantity is changed when creating a command from the backend that this watcher will trigger?

## nt.number theory – Why are \$ e \$ and \$ pi \$ close to the number 3?

I was just wondering if there was a deep reason for number theory for which $$e$$, in particular, is close to 3, rather than another integer. For example, there may be a counterpart of the identity of Euler in a finite field, where the "3" of the analog of $$e$$ is more inevitably transparent.

(This question is brought to you via "The Entropy Church".)

## Current number of proxies / threads to use to avoid proxy suspension?

Proxies … in 2 parts
1- It's been a long time since I did not do any major scratching … Can any one save me time and temporarily ban the proxies, and indicate the current proxy thread and the delay that I have to use in Scrapebox in order to get maximum scraping done without my proxies being banned?
2- If you have good marks for the private / exclusive agents, I am all ears
Thank you all

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